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_test.go

@@ -52,6 +52,64 @@ func TestSendEmailCodeHandlerSuccess(t *testing.T) {
 	assert.Equal(t, PublicRouteClassPublicAuth, authService.sendEmailCodeRouteClass)
 }
 
+func TestSendEmailCodeHandlerBodyLocaleOverridesHeader(t *testing.T) {
+	t.Parallel()
+
+	authService := &recordingAuthServiceClient{
+		sendEmailCodeResult: SendEmailCodeResult{
+			ChallengeID: "challenge-456",
+		},
+	}
+	handler := newPublicHandler(ServerDependencies{AuthService: authService})
+
+	req := httptest.NewRequest(
+		http.MethodPost,
+		"/api/v1/public/auth/send-email-code",
+		strings.NewReader(`{"email":"pilot@example.com","locale":"ru"}`),
+	)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept-Language", "fr-FR")
+	recorder := httptest.NewRecorder()
+
+	handler.ServeHTTP(recorder, req)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+	assert.Equal(t, SendEmailCodeInput{
+		Email:             "pilot@example.com",
+		Locale:             "ru",
+		PreferredLanguage: "ru",
+	}, authService.sendEmailCodeInput)
+}
+
+func TestSendEmailCodeHandlerEmptyBodyLocaleFallsBackToHeader(t *testing.T) {
+	t.Parallel()
+
+	authService := &recordingAuthServiceClient{
+		sendEmailCodeResult: SendEmailCodeResult{
+			ChallengeID: "challenge-789",
+		},
+	}
+	handler := newPublicHandler(ServerDependencies{AuthService: authService})
+
+	req := httptest.NewRequest(
+		http.MethodPost,
+		"/api/v1/public/auth/send-email-code",
+		strings.NewReader(`{"email":"pilot@example.com","locale":"   "}`),
+	)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept-Language", "ru-RU")
+	recorder := httptest.NewRecorder()
+
+	handler.ServeHTTP(recorder, req)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+	assert.Equal(t, SendEmailCodeInput{
+		Email:             "pilot@example.com",
+		Locale:             "   ",
+		PreferredLanguage: "ru-RU",
+	}, authService.sendEmailCodeInput)
+}
+
 func TestConfirmEmailCodeHandlerSuccess(t *testing.T) {
 	t.Parallel()