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/features/settings/general/General.tsx

@@ -114,6 +114,7 @@ import { SequenceCardStyle } from '../styles.css';
 import { UpdateProgress, useTauriUpdater } from '../../../hooks/useTauriUpdater';
 import { describeUpdateError, manualDownloadUrl } from '../../../utils/updateErrors';
 import { isTauri as isTauriEnv, invokeTauri, tauriInvoke } from '../../../hooks/useTauri';
+import { useKeychainMirrorStatus } from '../../../state/keychainMirror';
 import { isSafeGlobalToggleKey } from '../../../hooks/useCallHotkeys';
 import { customWindowChromeAtom } from '../../../state/customWindowChrome';
 import { useDateFormatItems } from '../../../hooks/useDateFormat';
@@ -238,6 +239,27 @@ function AutostartSetting() {
   );
 }
 
+// [Gitea #105] Desktop: whether the login is also kept in the OS keychain.
+function KeychainMirrorSetting() {
+  const status = useKeychainMirrorStatus();
+  if (!isTauriEnv() || status.state === 'idle' || status.state === 'cleared') return null;
+  let description: string;
+  if (status.state === 'ok') {
+    description =
+      "A copy of your login is kept in the system keychain (Windows Credential Manager). A later update will keep it only there, out of the app's data folder.";
+  } else if (status.state === 'unsupported') {
+    description =
+      "Not available on this system yet. Your login is saved in the app's data folder, as before.";
+  } else {
+    description = `Couldn't save a copy to the system keychain (${status.error}). You stay logged in; your login is saved in the app's data folder, as before.`;
+  }
+  return (
+    <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+      <SettingTile title="Login in the system keychain" description={description} />
+    </SequenceCard>
+  );
+}
+
 type ThemeSelectorProps = {
   themeNames: Record<string, string>;
   themes: Theme[];
@@ -570,6 +592,7 @@ function Appearance() {
 
       <DesktopChromeSetting />
       <AutostartSetting />
+      <KeychainMirrorSetting />
 
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile