feat(desktop): mirror the login tokens into the OS keychain (#105, step 1)

Step 1 of moving the desktop app's login out of the webview's plaintext
localStorage: keep a verified copy of the tokens in the OS keychain
(Windows Credential Manager, via cinny-desktop's new secure_session_*
commands). The session is still read from localStorage exactly as before,
so nothing about login changes and a keychain problem can't log anyone out.
Step 2 (a later release, once this has run on real installs) switches reads
to the keychain and drops the tokens from localStorage.

- sessions.ts: onSessionPersisted — listeners told about every session
  write (login, token rotation) and removal (logout); a throwing listener
  can't break the write.
- keychainMirror.ts: desktop only. Mirrors userId/deviceId/accessToken/
  refreshToken (not the rest of the session); reads first and writes only
  when the copy differs, then verifies by reading back; serialized, 5 s
  timeouts; no session → clear (also covers a logout whose reload beat the
  clear). Every failure is a status, never an exception. A desktop build
  without the commands reads as "unsupported", so this can ship before the
  desktop side.
- Settings → General (desktop): "Login in the system keychain" status.

Tested: unit tests (fake keychain: store, no rewrite when current, rotation,
clear, unsupported, missing commands, denied write, read-back mismatch,
timeout); a simulated desktop app with a fake keychain, 14/14 (login
mirrors only the secrets, Settings status, reload verifies without
rewriting, logout clears, an existing session is mirrored after upgrade,
Linux/denied show an honest status and stay logged in); the real Linux
desktop binary (commands answer "unsupported", login unaffected, Settings
says so). Unit 1295 pass, Playwright 20 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/sessions.ts

@@ -233,6 +233,27 @@ export type SessionStoreName = {
 //   crypto: 'crypto-store',
 // } as const;
 
+// [Gitea #105] Listeners told about every session write in THIS tab (login,
+// token rotation) and removal (logout), e.g. the desktop keychain mirror.
+// A throwing listener never breaks the write.
+type PersistListener = (session: Session | null) => void;
+const persistListeners = new Set<PersistListener>();
+const notifyPersisted = (session: Session | null): void => {
+  persistListeners.forEach((cb) => {
+    try {
+      cb(session);
+    } catch {
+      /* listener errors must not affect login/logout */
+    }
+  });
+};
+export const onSessionPersisted = (cb: PersistListener): (() => void) => {
+  persistListeners.add(cb);
+  return () => {
+    persistListeners.delete(cb);
+  };
+};
+
 // Persist the session. Writes the atomic blob FIRST (so the consistent,
 // never-torn copy is established before the multi-key legacy write), then
 // dual-writes the legacy keys for rollback safety. Signature is unchanged —
@@ -249,6 +270,7 @@ export function setFallbackSession(
   localStorage.setItem(SESSION_BLOB_KEY, JSON.stringify(persisted));
   // Dual-write the legacy keys (removal of this half is a future release).
   writeLegacyKeys(persisted);
+  notifyPersisted(sessionFromPersisted(persisted));
 }
 
 // Clear BOTH the atomic blob and every legacy key so no reader (blob-preferring
@@ -257,6 +279,7 @@ export const removeFallbackSession = () => {
   localStorage.removeItem(SESSION_BLOB_KEY);
   Object.values(LEGACY_KEYS).forEach((key) => localStorage.removeItem(key));
   Object.values(OIDC_KEYS).forEach((key) => localStorage.removeItem(key));
+  notifyPersisted(null);
 };
 
 // Read the session, preferring the atomic blob. If the blob is absent or