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/xiaomi-mimo.js

@@ -0,0 +1,125 @@
+/**
+ * Xiaomi MiMo usage — weekly quota from the Xiaomi account session.
+ *
+ * Primary path: GET {mimo-server}/api/user/usage authorized by the account-session
+ * cookie (see shared/mimoAccount.js). Response: { code: 0, data: { percent (remaining
+ * %), resetDate, resetAt } }.
+ *
+ * Fallback: the sk- API key cannot read the quota, so when no account session is
+ * available we surface a graceful message instead of failing.
+ */
+
+import { proxyAwareFetch } from "../../utils/proxyFetch.js";
+import { getMimoAccountUsage } from "../../shared/mimoAccount.js";
+
+const USAGE_URL = "https://aistudio.xiaomimimo.com/open-apis/v1/user/usage";
+
+/**
+ * @param {string|null|undefined} accessToken - sk- API key
+ * @param {object|null} providerSpecificData - may contain mimoPassToken, uid, etc.
+ * @param {object|null} proxyOptions
+ */
+export async function getXiaomiMimoUsage(accessToken = null, providerSpecificData = null, proxyOptions = null) {
+  // Preferred path: the weekly quota comes from the account service session
+  // (mimo-server /api/user/usage), which the sk- key cannot reach. The session is
+  // derived from MiMo Desktop's persisted passToken via the SSO/sts handshake.
+  const account = await getMimoAccountUsage(providerSpecificData, proxyOptions);
+  if (typeof account.percent === "number" && Number.isFinite(account.percent)) {
+    const remaining = Math.max(0, Math.min(100, Math.round(account.percent)));
+    const used = 100 - remaining;
+    let resetAt = null;
+    if (typeof account.resetAt === "number" && account.resetAt > 0) {
+      resetAt = new Date(account.resetAt * 1000).toISOString();
+    } else if (typeof account.resetDate === "string") {
+      const parsed = new Date(`${account.resetDate}T00:00:00Z`);
+      if (!Number.isNaN(parsed.getTime())) resetAt = parsed.toISOString();
+    }
+    return {
+      plan: "Xiaomi MiMo Desktop",
+      quotas: {
+        Weekly: { used, total: 100, remainingPercentage: remaining, resetAt, unlimited: false },
+      },
+    };
+  }
+
+  // Fallback: no account session available (Desktop never logged in, or its cookie
+  // store is locked). The sk- key cannot read the quota, so surface a clear message.
+  const key = accessToken || providerSpecificData?.apiKey;
+  if (!key || typeof key !== "string" || !key.trim()) {
+    return { message: "Xiaomi MiMo Desktop not connected. Add credentials to view usage." };
+  }
+
+  try {
+    const response = await proxyAwareFetch(
+      USAGE_URL,
+      {
+        method: "GET",
+        headers: {
+          Authorization: `Bearer ${key.trim()}`,
+          "X-Mimo-Source": "mimocode-cli",
+          Accept: "application/json",
+        },
+        signal: AbortSignal.timeout(10000),
+      },
+      proxyOptions,
+    );
+
+    if (response.status === 401) {
+      return {
+        plan: "Xiaomi MiMo Desktop",
+        message: "Weekly quota requires Xiaomi account session. API key alone is insufficient.",
+      };
+    }
+
+    if (!response.ok) {
+      return {
+        plan: "Xiaomi MiMo Desktop",
+        message: `Usage API error (${response.status})`,
+      };
+    }
+
+    const data = await response.json().catch(() => null);
+    if (!data || data.code !== 0 || !data.data) {
+      return {
+        plan: "Xiaomi MiMo Desktop",
+        message: "Usage endpoint returned unexpected response.",
+      };
+    }
+
+    const { percent, resetDate } = data.data;
+    if (typeof percent !== "number" || !Number.isFinite(percent)) {
+      return {
+        plan: "Xiaomi MiMo Desktop",
+        message: "Usage data missing percent field.",
+      };
+    }
+
+    // percent = remaining percentage (e.g. 94 means 94% remaining)
+    const remaining = Math.max(0, Math.min(100, Math.round(percent)));
+    const used = 100 - remaining;
+
+    // Parse resetDate — expected format "2026-09-16"
+    let resetAt = null;
+    if (resetDate && typeof resetDate === "string") {
+      const parsed = new Date(`${resetDate}T00:00:00Z`);
+      if (!Number.isNaN(parsed.getTime())) {
+        resetAt = parsed.toISOString();
+      }
+    }
+
+    return {
+      plan: "Xiaomi MiMo Desktop",
+      quotas: {
+        Weekly: {
+          used,
+          total: 100,
+          remainingPercentage: remaining,
+          resetAt,
+          unlimited: false,
+        },
+      },
+    };
+  } catch (error) {
+    return { message: `Xiaomi MiMo Desktop usage error: ${error.message}` };
+  }
+}