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.

src/lib/db/driver.js

@@ -19,6 +19,10 @@ async function tryBunSqlite() {
 async function tryBetterSqlite() {
   // Skip on Bun — better-sqlite3 native bindings unsupported
   if (process.versions.bun) return null;
+  // Skip on Node >= 24: the native addon SIGSEGVs on load there, which is a
+  // process-level crash the try/catch below cannot recover from. node:sqlite covers it.
+  const [nodeMajor] = process.versions.node.split(".").map(Number);
+  if (nodeMajor >= 24) return null;
   try {
     const { createBetterSqliteAdapter } = await import("./adapters/betterSqliteAdapter.js");
     return createBetterSqliteAdapter(DATA_FILE);