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/openapi.yaml

@@ -134,10 +134,12 @@ paths:
         that must later be confirmed through
         `POST /api/v1/public/auth/confirm-email-code`.
 
-        The JSON body stays unchanged. Callers may additionally supply the
-        standard `Accept-Language` header so the gateway can derive the
-        auth-mail locale and first-login preferred-language candidate. Missing
-        or unsupported values fall back to `en`.
+        Callers select the auth-mail locale through the optional
+        `locale` field on the JSON body, which takes priority over the
+        request `Accept-Language` header. The body field is the canonical
+        channel because Safari silently drops JS-set `Accept-Language`
+        headers; non-Safari clients can still rely on the header alone.
+        Missing or unsupported values fall back to `en`.
 
         This route is unauthenticated and classified as `public_auth`.
         Public REST anti-abuse applies a per-IP bucket derived from
@@ -302,6 +304,16 @@ components:
           type: string
           description: Single client e-mail address that should receive the login code.
           format: email
+        locale:
+          type: string
+          description: |
+            Optional BCP 47 language tag the caller prefers for the
+            delivered code. 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.
+            Empty / malformed values fall back to the header, which
+            in turn falls back to `en`.
     SendEmailCodeResponse:
       type: object
       additionalProperties: false