fix(quick-261029-d9j): pick the mail language from Accept-Language, add mail.fallback_locale, Polish subjects

- preferred_locale and the request locale are normalised (comma lists,
  q-weights, region subtags, case) to pl or en by NormalizeMailLocale, so a
  raw Accept-Language such as en-GB,en;q=0.9 selects the -en templates
- new golem15.user.mail.fallback_locale (default pl) is used when neither
  names a supported language; ResolveMailLocaleFor applies it, and Boot
  rejects any value other than pl or en
- activate, reactivate and restore Polish templates get Polish subjects
- no template, route or payload changes; MailTemplate and
  ResolveMailLocale keep their signatures

classes/mail.go

@@ -2,27 +2,128 @@ package classes
 
 import (
 	"context"
+	"fmt"
+	"strconv"
+	"strings"
 
 	"git.golem15.com/golem15/sm-user-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/towel"
 )
 
-// MailTemplate appends -en for English and leaves every other locale,
-// including empty and pl, on the unsuffixed Polish template name.
+const (
+	// mailFallbackLocaleKey names the mail language used when neither the
+	// user nor the request names one the plugin has mail for.
+	mailFallbackLocaleKey = "golem15.user.mail.fallback_locale"
+	// mailLocalePL is the unsuffixed (Polish) template set and the default
+	// fallback.
+	mailLocalePL = "pl"
+	// mailLocaleEN is the -en template set.
+	mailLocaleEN = "en"
+)
+
+// supportedMailLocale reports whether the plugin has mail in lang.
+func supportedMailLocale(lang string) bool {
+	return lang == mailLocalePL || lang == mailLocaleEN
+}
+
+// NormalizeMailLocale reduces a locale value to the language the plugin has
+// mail for: pl, en or "". It accepts a plain code (en, PL), a region form
+// (en-GB, pl_PL) and an Accept-Language list with q-weights
+// (en-GB,en;q=0.9). Entries whose q does not parse, lies outside 0..1 or is
+// 0 are dropped, as are languages other than pl and en (the wildcard
+// included); of the rest the one with the highest q wins, ties going to the
+// earliest. The result never holds anything but pl, en or "".
+func NormalizeMailLocale(raw string) string {
+	best, bestQ := "", -1.0
+	for _, entry := range strings.Split(raw, ",") {
+		parts := strings.Split(entry, ";")
+		tag := strings.ToLower(strings.TrimSpace(parts[0]))
+		if i := strings.IndexAny(tag, "-_"); i >= 0 {
+			tag = tag[:i]
+		}
+		if !supportedMailLocale(tag) {
+			continue
+		}
+		q, ok := 1.0, true
+		for _, param := range parts[1:] {
+			param = strings.TrimSpace(param)
+			if len(param) < 2 || !strings.EqualFold(param[:2], "q=") {
+				continue
+			}
+			v, err := strconv.ParseFloat(strings.TrimSpace(param[2:]), 64)
+			if err != nil || v < 0 || v > 1 {
+				ok = false
+				break
+			}
+			q = v
+		}
+		if !ok || q == 0 {
+			continue
+		}
+		if q > bestQ {
+			best, bestQ = tag, q
+		}
+	}
+	return best
+}
+
+// MailTemplate appends -en when locale normalises to English
+// (NormalizeMailLocale) and leaves every other value, empty included, on the
+// unsuffixed Polish template name. Callers that want the application's
+// configured fallback resolve the locale with ResolveMailLocaleFor first.
 func MailTemplate(base, locale string) string {
-	if locale == "en" {
+	if NormalizeMailLocale(locale) == mailLocaleEN {
 		return base + "-en"
 	}
 	return base
 }
 
-// ResolveMailLocale prefers the user's saved locale, then the request locale.
+// ResolveMailLocale prefers the user's saved locale, then the request locale,
+// each normalised by NormalizeMailLocale; a value naming neither pl nor en is
+// skipped. It returns pl, en or "" (neither names a supported language).
 func ResolveMailLocale(ctx context.Context, user *models.User) string {
-	if user != nil && user.PreferredLocale != nil && *user.PreferredLocale != "" {
-		return *user.PreferredLocale
+	if user != nil && user.PreferredLocale != nil {
+		if loc := NormalizeMailLocale(*user.PreferredLocale); loc != "" {
+			return loc
+		}
 	}
-	if loc, ok := towel.Locale(ctx); ok {
-		return loc
+	if ctx != nil {
+		if raw, ok := towel.Locale(ctx); ok {
+			return NormalizeMailLocale(raw)
+		}
 	}
 	return ""
 }
+
+// MailFallbackLocale returns golem15.user.mail.fallback_locale normalised to
+// pl or en: the mail language when neither the user nor the request names
+// one the plugin has mail for. The default is pl (the unsuffixed
+// templates); an English-only site sets en, for example through
+// SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en. A missing app, config or
+// blank value gives pl; any other value gives pl and an error naming the key.
+func MailFallbackLocale(app *backpack.App) (string, error) {
+	if app == nil || app.Config == nil {
+		return mailLocalePL, nil
+	}
+	raw := strings.TrimSpace(app.Config.String(mailFallbackLocaleKey))
+	if raw == "" {
+		return mailLocalePL, nil
+	}
+	if loc := NormalizeMailLocale(raw); loc != "" {
+		return loc, nil
+	}
+	return mailLocalePL, fmt.Errorf("golem15.user: %s must be pl or en, got %q", mailFallbackLocaleKey, raw)
+}
+
+// ResolveMailLocaleFor returns the language to mail user in, always pl or
+// en: ResolveMailLocale, then the application's MailFallbackLocale. It is the
+// helper other plugins use to mail in the user's language with the
+// application's fallback; an invalid fallback (rejected at boot) counts as pl.
+func ResolveMailLocaleFor(ctx context.Context, app *backpack.App, user *models.User) string {
+	if loc := ResolveMailLocale(ctx, user); loc != "" {
+		return loc
+	}
+	loc, _ := MailFallbackLocale(app)
+	return loc
+}