feat: H1 to H2 setting with the admin save conversion (tracer)

- content_h1_to_h2 and generic_anchors on the settings row (migration
  202610280001), Headings and links tab, config key
  golem15.journal.content_h1_to_h2 used when no settings row exists
- classes.ConvertH1 on the x/net/html tokenizer; the Blog Editor types
  convert while the setting is on and answer editor.h1_converted
- GET /posts/{slug} reports content_h1_to_h2 (additive key)

plugin.go

@@ -25,6 +25,7 @@ import (
 
 var (
 	_ party.Plugin        = (*Plugin)(nil)
+	_ pact.HasConfig      = (*Plugin)(nil)
 	_ pact.HasMigrations  = (*Plugin)(nil)
 	_ pact.HasModels      = (*Plugin)(nil)
 	_ pact.HasLang        = (*Plugin)(nil)
@@ -33,6 +34,9 @@ var (
 	_ surf.BucketProvider = (*Plugin)(nil)
 )
 
+//go:embed config
+var configFS embed.FS
+
 //go:embed lang
 var langFS embed.FS
 
@@ -104,6 +108,9 @@ func (p *Plugin) wireSiteCache(app *backpack.App) error {
 	return nil
 }
 
+// ConfigFS ships config/config.yaml (golem15.journal.content_h1_to_h2).
+func (p *Plugin) ConfigFS() fs.FS { return configFS }
+
 func (p *Plugin) LangFS() fs.FS { return langFS }
 
 func (p *Plugin) Migrations() []*gormigrate.Migration { return updates.All() }