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>

ui/README.md

@@ -64,6 +64,7 @@ ui/
 ├── buf.gen.yaml            local-plugin TS Protobuf-ES generator
 ├── docs/                    topic-based design notes
 │   ├── auth-flow.md         email-code login, session store, revocation
+│   ├── i18n.md              translation primitive, native-name picker, extensibility
 │   ├── storage.md           web KeyStore/Cache, IDB schema, baseline
 │   ├── testing.md           per-PR / release test tiers
 │   └── wasm-toolchain.md    TinyGo build, JSDOM loading, bundle budget
@@ -83,6 +84,8 @@ Linked topic docs:
 
 - [`docs/auth-flow.md`](docs/auth-flow.md) — email-code login,
   session store state machine, revocation watcher.
+- [`docs/i18n.md`](docs/i18n.md) — translation primitive, native-name
+  language picker, recipe for adding a new locale.
 - [`docs/storage.md`](docs/storage.md) — web KeyStore/Cache,
   IndexedDB schema, browser baseline.
 - [`docs/wasm-toolchain.md`](docs/wasm-toolchain.md) — TinyGo build,