feat(auth): M9 Part 2 — native in-app SSO via the mobile bridge

Add the app client for the Mobile SSO Authorization Bridge (PLAN.md §4.2):
native "Sign in with <provider>" without shipping any OAuth secret.

- Pkce: pure-JVM RFC 7636 S256 verifier/challenge + CSRF state, encoded to
  match the backend's base64url(SHA-256) exactly.
- SsoAuthManager (Singleton): mints PKCE+state, builds the /auth/mobile/sso/start
  URL for a Custom Tab, verifies the returned state, exchanges the one-time code
  with the stashed verifier, and drives the existing SessionManager.onSignedIn —
  no new token-storage or refresh code. Pending flow is in-memory (fails closed on
  process death). Exposes an outcome StateFlow the login screen consumes.
- SsoApi + DTOs: GET /auth/providers discovery and POST /auth/mobile/sso/exchange
  (tagged NO_SESSION so a credential 401 isn't read as an expired session).
- MainActivity: runicgateway://auth/callback intent-filter + singleTop; parses the
  callback Uri (the Android edge) and hands raw params to SsoAuthManager.
- LoginScreen/ViewModel: render a button per discovered provider, opening the
  bridge in a Custom Tab; fall back to the website login hand-off when none.

Additive — no other screen's data flow changes; no backend work. Custom scheme
only for now (App Links deferred, APP_LINKS.md).

Tests (JVM, +14): Pkce vector/charset, start-URL building, and the full
complete() flow over a fake SsoApi + real SessionManager (success signs in;
state mismatch / missing pending fail without exchanging; error callback →
declined; 401 → expired-code; replay finds no pending).

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NgyHnrNa8WwG3doxvxjuCr

app/src/main/AndroidManifest.xml

@@ -24,14 +24,32 @@
         android:supportsRtl="true"
         android:theme="@style/Theme.RunicGateway">
 
+        <!-- singleTop so the SSO Custom Tab returning via the deep link reuses the
+             running task (onNewIntent) instead of stacking a second activity. -->
         <activity
             android:name=".MainActivity"
             android:exported="true"
+            android:launchMode="singleTop"
             android:theme="@style/Theme.RunicGateway">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
+
+            <!-- Native SSO callback (M9, PLAN.md §4.2). The bridge deep-links the
+                 one-time authorization code back to this fixed, app-owned custom
+                 scheme; it must match SsoAuthManager.REDIRECT_URI and the backend's
+                 MOBILE_AUTH_REDIRECT_URIS allowlist exactly. Custom scheme only for
+                 now — HTTPS App Links are deferred (docs/android/APP_LINKS.md). -->
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data
+                    android:scheme="runicgateway"
+                    android:host="auth"
+                    android:path="/callback" />
+            </intent-filter>
         </activity>
 
         <!-- The embedded distributor's persistent ntfy connection (M7, PLAN.md §11).