feat(account): VK ID web login to link a VK identity from a browser

A browser has no signed VK Mini App launch params, so linking VK on the web uses
VK ID's raw OAuth 2.1 flow (PKCE, no @vkid/sdk): the SPA redirects to VK's hosted
login and returns with an authorization code, which the gateway exchanges
server-side (confidential, under the VK "Web" app's protected key) for the trusted
vk user id — then the existing link/merge machinery attaches or merges it.

- fbs LinkVKRequest{code, device_id, code_verifier}; codec + TS bindings.
- backend link.Service ConfirmVK/MergeVK/attachVK (KindVK, mirror Telegram),
  handleLinkVK[Merge], routes /user/link/vk[/merge], backendclient LinkVK[Merge].
- gateway internal/vkid confidential code exchange (id.vk.com/oauth2/auth);
  transcode link.vk.confirm/merge (registered only when configured) + config
  GATEWAY_VK_ID_{APP_ID,CLIENT_SECRET,REDIRECT_URL} + main wiring.
- UI lib/vkid (PKCE authorize redirect + callback), Profile "Link VK" control,
  boot callback handling; a merge re-authorizes for a fresh code (VK codes are
  single-use). Web-only (a redirect strands a Mini App webview).
- Deploy: VITE_VK_APP_ID + VITE_VK_ID_REDIRECT_URL build args + gateway env,
  ci.yaml/prod-deploy TEST_/PROD_ vars, compose/Dockerfile/.env.example/README.
- Tests: vkid exchange unit (string/number user_id, id_token fallback, errors),
  transcode link.vk, backend ConfirmVK/MergeVK inttest, codec encodeLinkVK.
- Docs: ARCHITECTURE §4, FUNCTIONAL(+ru), gateway README.

ui/src/lib/app.svelte.ts

@@ -34,6 +34,7 @@ import {
   telegramCloudSet,
 } from './telegram';
 import { onVKPath, insideVK, vkInit, vkClose, vkDisableSwipeBack, vkLaunchParams, vkUserName, vkStartParam, vkOnScheme, vkOnInsets, vkSetViewSettings, appearanceForBg } from './vk';
+import { pendingVKLink, type VKLinkCallback } from './vkid';
 import { haptic } from './haptics';
 import { CLOUD_PREFS_KEY, decodeClientPrefs, encodeClientPrefs } from './cloudprefs';
 import { parseStartParam } from './deeplink';
@@ -96,6 +97,10 @@ export const app = $state<{
   /** Set once the account has been deleted: App.svelte shows the terminal "account deleted"
    *  screen and all push/poll is stopped. Reopening the app just creates a fresh account. */
   accountDeleted: boolean;
+  /** A VK ID web-link authorization callback captured on boot (see lib/vkid): the browser
+   *  returned from VK with an auth code. Profile consumes it on mount to finish the link or
+   *  merge (a full-page redirect loses the route, so boot routes here). Null on a normal load. */
+  vkLinkPending: VKLinkCallback | null;
   toast: Toast | null;
   lastEvent: PushEvent | null;
   theme: ThemePref;
@@ -145,6 +150,7 @@ export const app = $state<{
   profile: null,
   blocked: null,
   accountDeleted: false,
+  vkLinkPending: null,
   toast: null,
   lastEvent: null,
   theme: 'auto',
@@ -778,9 +784,19 @@ export async function bootstrap(): Promise<void> {
   }
 
   const saved = await loadSession();
+  // A VK ID web-link callback (?code&device_id&state) rides the URL after the redirect back
+  // from VK; capture and clear it here (it needs the restored session to link against).
+  const vkcb = pendingVKLink();
   if (saved) {
     await adoptSession(saved);
-    if (router.route.name === 'login') navigate('/');
+    if (vkcb) {
+      // The full-page redirect lost the in-app route, so hand the callback to Profile, which
+      // finishes the link or merge on mount.
+      app.vkLinkPending = vkcb;
+      navigate('/profile');
+    } else if (router.route.name === 'login') {
+      navigate('/');
+    }
   } else if (router.route.name !== 'login' && router.route.name !== 'confirm') {
     navigate('/login');
   }