feat(xiaomi-mimo): merge MiMo Desktop support into xiaomi-mimo as dual auth

Adds the Desktop-exclusive Preview models and the Xiaomi account-session
route to the existing xiaomi-mimo provider instead of a separate
xiaomi-desktop provider, so the dashboard shows one MiMo entry rather than
three overlapping ones.

Dual auth, same pattern as kimi — API key (sk-) covers the cloud API,
Desktop/OAuth adds the account session used by the Preview models:

- registry: category oauth, authModes [oauth, apikey], oauth block, the two
  mimo-x-*-preview models, and the invite signupUrl
- executor: routes Preview models to the account-service route with a Cookie
  session, everything else keeps the sourceFormat-matched transport
- oauth: custom ECDH encrypted-callback flow (X25519 -> SHA256 -> AES-256-GCM)
  with a loopback callback proxy, plus one-click import of the local Desktop
  auth.json
- usage: weekly quota from the account session

Fixes found while merging:

- the OAuth browser flow was dead: poll-status cleared the session before the
  client could POST /exchange, so every exchange returned 400
- a Claude-format client was sent to /v1/chat/completions instead of the
  declared /anthropic/v1/messages transport, because buildUrl ignored
  runtimeTransport
- stopXiaomiMimoProxy leaked every pending session (each holding an X25519
  private key) for the process lifetime
- the OAuth exchange did not persist the Desktop passToken, so the Preview
  models could never work after a browser sign-in

Removes dead code: the local engine token minting (mimoEngine, never called
on the request path), the model-catalog and usage routes, engineToken/
engineUrl plumbing, and an unread top-level usage block.

Adds tests/unit/xiaomi-mimo-{executor,oauth-session,oauth-proxy}.test.js —
the provider previously had none.

open-sse/executors/index.js

@@ -17,6 +17,7 @@ import { PerplexityWebExecutor } from "./perplexity-web.js";
 import { OllamaLocalExecutor } from "./ollama-local.js";
 import { CommandCodeExecutor } from "./commandcode.js";
 import { XiaomiTokenplanExecutor } from "./xiaomi-tokenplan.js";
+import { XiaomiMimoExecutor } from "./xiaomi-mimo.js";
 import { MimoFreeExecutor } from "./mimo-free.js";
 import { CodeBuddyExecutor } from "./codebuddy-cn.js";
 import { CodeBuddyIntlExecutor } from "./codebuddy-intl.js";
@@ -50,6 +51,7 @@ const executors = {
   "ollama-local": new OllamaLocalExecutor(),
   commandcode: new CommandCodeExecutor(),
   "xiaomi-tokenplan": new XiaomiTokenplanExecutor(),
+  "xiaomi-mimo": new XiaomiMimoExecutor(),
   "mimo-free": new MimoFreeExecutor(),
   mmf: new MimoFreeExecutor(), // Alias for mimo-free
   "codebuddy-cn": new CodeBuddyExecutor(),
@@ -93,6 +95,7 @@ export { PerplexityWebExecutor } from "./perplexity-web.js";
 export { OllamaLocalExecutor } from "./ollama-local.js";
 export { CommandCodeExecutor } from "./commandcode.js";
 export { XiaomiTokenplanExecutor } from "./xiaomi-tokenplan.js";
+export { XiaomiMimoExecutor } from "./xiaomi-mimo.js";
 export { MimoFreeExecutor } from "./mimo-free.js";
 export { CodeBuddyExecutor } from "./codebuddy-cn.js";
 export { CodeBuddyIntlExecutor } from "./codebuddy-intl.js";