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/services/usage.js

@@ -17,6 +17,7 @@ import { getDeepseekUsage } from "./usage/deepseek.js";
 import { getOpenCodeGoUsage } from "./usage/opencode-go.js";
 import { getGroqUsage } from "./usage/groq.js";
 import { getZedUsage } from "./usage/zed.js";
+import { getXiaomiMimoUsage } from "./usage/xiaomi-mimo.js";
 import { resolveQoderCredentials } from "./qoderModels.js";
 import { getGlmUsage } from "./usage/glm.js";
 import {
@@ -60,6 +61,7 @@ const USAGE_HANDLERS = {
   deepseek: (c) => getDeepseekUsage(c.apiKey, c.proxyOptions),
   groq: (c) => getGroqUsage(c.apiKey, c.proxyOptions),
   zed: (c) => getZedUsage(c.accessToken, c.providerSpecificData, c.proxyOptions),
+  "xiaomi-mimo": (c) => getXiaomiMimoUsage(c.accessToken, c.providerSpecificData, c.proxyOptions),
 };
 
 export async function getUsageForProvider(connection, proxyOptions = null, options = {}) {