feat(12.1-02): password and form-only fields, rules per operation and preset

- pact.FormVirtualFields lists form fields that are not model columns: never
  bound, filled or projected; their values reach the Form hooks through
  cabana.VirtualFieldsFromContext when the field's context allows the operation
- type: password is a masked field that must be listed as virtual
- pact.FormRules supplies the rule set per operation and replaces the model's
  Rules() for admin saves; a rule on a virtual field sees the submitted value
- preset on a text field follows another text field on the create form
- SPA: PasswordField, preset handling in FormView, empty password left out of
  an update
- README, docs, OpenAPI document, TS types and dist updated

modules/cabana/README.md

@@ -21,6 +21,8 @@ Schema-driven admin backend that compiles WinterCMS-style YAML list, form, filte
 - Row state: a controller implementing `pact.ListRowStates` is called once per list page with the page's records and the list's database handle. The list response carries `meta.row_states`, keyed by row id, with values from the fixed set `deleted`, `negative`, `disabled` in that order; a value outside the set is dropped and logged, rows without a state are left out, and a controller without the hook sends no `row_states` key. The badge texts are the list messages `rowStateDeleted`, `rowStateNegative` and `rowStateDisabled`, defaulting to `backend::lang.messages.list.row_state_*`. A soft-deleted record that the controller's `pact.ListExtendQuery` and `pact.FormExtendQuery` include can be shown, updated (it stays soft-deleted), targeted by bulk and record actions and removed for good by the controller's `pact.FormAfterDelete`.
 - Record actions: `recordActions` in `config_form.yaml` lists names the controller registers through `pact.HasAdminRecordActions`, a third action namespace with the same reserved names. It needs the form's `preview` block: record actions are offered on the preview screen, and a form that declares them without one fails boot. The show response's `meta.actions` (`cabana.RecordAction` entries with localized `label` and `confirm`) carries only the declared actions the requesting administrator may run and whose `Applies` reports true for the record; the key is absent when none is offered, and create and update responses never carry it. The action route loads the record through `pact.FormExtendQuery` with a row lock in one transaction (one 404 for a missing and an out-of-scope id), checks `Applies` again (409 when it reports false) and then runs the action. An unknown or duplicate name, or an action without a label, fails boot. Each run is logged with the controller, action, administrator and record id.
 - Preview screen: a `preview` mapping in `config_form.yaml` (`preview: {}`, or with `headerPartial: <name>` for a status hint) gives the form a read-only record screen in the admin SPA. The form schema reports it as `preview` (`cabana.FormPreview`), fields with `context: preview` are shown only there and are never written by a save, `messages.preview` and `messages.edit` name the screen's subtitle and edit button, and `recordUrl` and the form redirects may point at it as `.../preview/:id`. An empty `preview:` key or an unknown key inside it fails boot.
+- Form-only fields: a controller implementing `pact.FormVirtualFields` lists fields of its `fields.yaml` that are not columns of the form. They are exempt from the column binding, never filled into the model and never part of a record response; the values an administrator submits reach the Form hooks through `cabana.VirtualFieldsFromContext`, only for fields whose `context` allows the operation, and a nested value is a 422 on the field. `type: password` is a masked field that must be listed this way, so a password is sent in a save body and never comes back. A controller implementing `pact.FormRules` supplies the validation rules per operation (`create` or `update`), which replace the model's `Rules()` for admin saves; a rule on a virtual field is checked against the submitted value, never against a model column of the same name. Neither is available on settings forms or relation forms.
+- Preset fields: `preset` on a `type: text` field (a source field name, or a mapping with `field` and `type`, `slug` or `exact`) makes the field follow another text field of the same form on the create screen until the administrator edits it. The schema reports it as `preset` (`cabana.FieldPreset`); the server does not fill the field.
 - Server-rendered partials: `headerPartial: <name>` in `config_list.yaml` (a strip above the list) and `type: partial` with `path: <name>` in `fields.yaml` render the template `{ConfigDir}/_<name>.htm` with `html/template` against a view model from the controller's `pact.AdminPartialData`. The result reaches the SPA as an allowlisted node tree, never as an HTML string. A missing or unparsable template, a free-form path or a controller without `pact.AdminPartialData` fails boot.
 - Date pickers: a `type: datepicker` field in `fields.yaml` edits a date (`mode: date`, a `lagoon.Date` column), a date and time (`mode: datetime`, the default, a `time.Time` column stored in UTC) or a time of day (`mode: time`, a `lagoon.TimeOfDay` column); pointers to the three types make the value optional. It accepts WinterCMS's `mode`, `format` (a PHP `date()` format, served also as `displayFormat` in the SPA's tokens), `minDate`, `maxDate`, `yearRange`, `firstDay`, `twelveHour` and `ignoreTimezone`; any other key, a format letter with no equivalent, bounds on `mode: time`, `ignoreTimezone` outside `mode: datetime` or a column whose Go type does not match the mode fails boot. The save rechecks `minDate` and `maxDate` on the calendar date and answers 422 on the field. List columns take `type: date` and `type: time` for these columns; when `type` is omitted, a `time.Time` column is compiled as `datetime`, a `lagoon.Date` column as `date` and a `lagoon.TimeOfDay` column as `time`. A struct column that implements `sql.Scanner` or `driver.Valuer` is never taken for a relation.
 - File uploads: a `type: fileupload` field in `fields.yaml` edits an attachOne or attachMany relation the record model declares through `attach.HasRelations` (its `AttachRelations` method) next to `attach.Owner`. The field accepts WinterCMS's `mode` (`image` or `file`), `fileTypes`, `mimeTypes`, `maxFilesize` (megabytes), `maxFiles` (attachMany only), `imageWidth`, `imageHeight`, `thumbOptions` (only `mode`: `auto`, `exact`, `crop` or `fit`), `useCaption` and `prompt`; any other key, an image-mode file type outside jpg, jpeg, png, gif and webp, a name that is not a declared relation or a `maxFilesize` whose file plus 64 KiB of multipart framing exceeds `http.body_limits.upload_bytes` fails boot. Uploads and removals are deferred, as in WinterCMS: the SPA sends a random form session key in the `X-Session-Key` header (`cabana.SessionKeyHeader`) with every file call and with the save, the server keeps the pending work in `deferred_bindings` against that key and the signed-in administrator, and the record's next create or update save applies it inside its transaction. A retry of the same upload may send `X-Upload-Id` so the server returns the already stored file. A save that fails with 422 keeps the pending uploads; another administrator's key matches nothing. The upload route caps the request body at the smaller of `http.body_limits.upload_bytes` and `maxFilesize` plus 64 KiB and answers 413 `payload_too_large` past it; the size, type and image checks run on the server (through `attach.Store`) and answer 422 on the field. A file list (`cabana.FileItem`) carries `url` and `thumb_url` only for a public relation.
@@ -206,6 +208,8 @@ func (p *Plugin) AdminFS() fs.FS { return adminFS }
 | `cabana.BackendUser` / `cabana.BackendUserRole` / `cabana.BackendUsers` | GORM models of the backend user tables and the principal loader used by the guard. |
 | `cabana.Allows` | Checks a principal against required permission codes. |
 | `cabana.TxFromContext` | The transaction a write route is running in, from the context of a lifecycle hook or scope. |
+| `cabana.VirtualFieldsFromContext` | The submitted values of the form's virtual fields (`pact.FormVirtualFields`), from the context of a Form hook during a create or update; a copy, keyed by field name. |
+| `cabana.FieldPreset` | A text field's `preset` in the form schema: the source field and the type, `slug` or `exact`. |
 | `cabana.WriteData` / `cabana.WriteError` / `cabana.WriteErrorDetails` | Write the admin success and error envelopes. |
 | `cabana.ValidationError` / `cabana.ListValidationError` | Field-level `validation_failed` errors. A `pact.AdminAction` may return a `cabana.ValidationError` to answer 422. |
 | `cabana.ForbiddenError` | A write controller code refuses: a hook or an action returns it and the route answers 403 `forbidden` with its localized `Message` and `Details`; the transaction is rolled back. |