phase 7+: i18n primitive + login language picker + autocomplete-off

Adds a minimal Svelte 5 i18n primitive (`src/lib/i18n/`) backing the
login form, the layout blocker page, and the lobby placeholder.
SUPPORTED_LOCALES drives both the picker and the runtime lookup;
adding a language is a two-step change inside `src/lib/i18n/`.

Login form gains a globe-icon language dropdown (English / Русский
in their native names), defaulting to navigator.languages with `en`
as the fallback. Switching the locale re-renders the form in place;
on submit, the locale rides in the JSON body of `send-email-code`
because Safari/WebKit silently drops JS-set Accept-Language. Gateway
gains a body `locale` field that takes priority over the request
header for preferred-language resolution.

Email and code inputs disable browser autofill / suggestions
(`autocomplete=off` + `autocorrect=off` + `autocapitalize=off` +
`spellcheck=false`) so Keychain / address-book pickers and
remembered-value dropdowns no longer fire on focus.

Cross-cuts:
- backend & gateway openapi: clarify that body `locale` is honored.
- docs/FUNCTIONAL{,_ru}.md §1.2: document body-vs-header priority.
- gateway tests: body `locale` overrides Accept-Language; blank
  body `locale` falls back to header.
- new ui/docs/i18n.md; cross-links from auth-flow.md and ui/README.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gateway/internal/restapi/public_auth.go

@@ -56,9 +56,16 @@ type SendEmailCodeInput struct {
 	// code challenge.
 	Email string `json:"email"`
 
-	// PreferredLanguage stores the canonical BCP 47 language tag derived from
-	// the public Accept-Language header for upstream auth-mail localization and
-	// create-only user registration context.
+	// Locale is the optional BCP 47 language tag the caller wants the
+	// auth-mail in. The body field is the canonical channel because Safari
+	// silently drops JS-set Accept-Language headers; when set, it overrides
+	// the request Accept-Language for preferred-language resolution.
+	Locale string `json:"locale,omitempty"`
+
+	// PreferredLanguage stores the canonical BCP 47 language tag derived
+	// from Locale (preferred) or the Accept-Language header (fallback) for
+	// upstream auth-mail localization and create-only user registration
+	// context.
 	PreferredLanguage string `json:"-"`
 }
 
@@ -209,7 +216,15 @@ func handleSendEmailCode(authService AuthServiceClient, timeout time.Duration) g
 			abortInvalidRequest(c, err.Error())
 			return
 		}
-		input.PreferredLanguage = resolvePreferredLanguage(c.Request.Header.Get("Accept-Language"))
+		// Body locale wins over the request header so Safari clients,
+		// which cannot set Accept-Language from JavaScript, can still
+		// pick a non-system mail language. Empty / malformed values
+		// fall through resolvePreferredLanguage to the default.
+		if strings.TrimSpace(input.Locale) != "" {
+			input.PreferredLanguage = resolvePreferredLanguage(input.Locale)
+		} else {
+			input.PreferredLanguage = resolvePreferredLanguage(c.Request.Header.Get("Accept-Language"))
+		}
 
 		callCtx, cancel := context.WithTimeout(c.Request.Context(), timeout)
 		defer cancel()