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/gen/fbs/scrabblefb/link-vkrequest.ts

@@ -0,0 +1,72 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+export class LinkVKRequest {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):LinkVKRequest {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsLinkVKRequest(bb:flatbuffers.ByteBuffer, obj?:LinkVKRequest):LinkVKRequest {
+  return (obj || new LinkVKRequest()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsLinkVKRequest(bb:flatbuffers.ByteBuffer, obj?:LinkVKRequest):LinkVKRequest {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new LinkVKRequest()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+code():string|null
+code(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+code(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
+deviceId():string|null
+deviceId(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+deviceId(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
+codeVerifier():string|null
+codeVerifier(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+codeVerifier(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
+static startLinkVKRequest(builder:flatbuffers.Builder) {
+  builder.startObject(3);
+}
+
+static addCode(builder:flatbuffers.Builder, codeOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(0, codeOffset, 0);
+}
+
+static addDeviceId(builder:flatbuffers.Builder, deviceIdOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(1, deviceIdOffset, 0);
+}
+
+static addCodeVerifier(builder:flatbuffers.Builder, codeVerifierOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(2, codeVerifierOffset, 0);
+}
+
+static endLinkVKRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createLinkVKRequest(builder:flatbuffers.Builder, codeOffset:flatbuffers.Offset, deviceIdOffset:flatbuffers.Offset, codeVerifierOffset:flatbuffers.Offset):flatbuffers.Offset {
+  LinkVKRequest.startLinkVKRequest(builder);
+  LinkVKRequest.addCode(builder, codeOffset);
+  LinkVKRequest.addDeviceId(builder, deviceIdOffset);
+  LinkVKRequest.addCodeVerifier(builder, codeVerifierOffset);
+  return LinkVKRequest.endLinkVKRequest(builder);
+}
+}