feat(sso): single "Sign in with SSO" button with a native provider picker

Collapse the per-provider login buttons into one "Sign in with SSO" entry. With a
single configured provider it launches straight through; with several it opens a
native ModalBottomSheet picker (driven by the discovery list the app already
fetches — no website chooser page, no Google SDK). Each row opens the Custom-Tab
bridge for that provider.

Also make the login screen dismiss reliably after any sign-in: the LOGIN
destination now pops as soon as the shared session becomes SignedIn, not only via
the login VM's local flag — the deep-link/recomposition timing of the Custom-Tab
return could otherwise leave the login screen up even though the session was
established.

Verified on emulator with two providers: the picker lists both, completing SSO via
one signs in and returns to Home (exchange 200, session persisted). lint + build green.

Co-Authored-By: Claude <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -142,7 +142,10 @@
     <string name="login_button">Sign in</string>
     <string name="login_register">Create an account</string>
     <string name="login_forgot">Forgot your password?</string>
-    <!-- %1$s is the provider name, e.g. "Google" or "Discord" (native SSO, M9). -->
+    <!-- Single SSO entry point; the picker lists the shard's providers (native SSO, M9/M10). -->
+    <string name="login_sso_button">Sign in with SSO</string>
+    <string name="login_sso_pick_title">Choose a sign-in provider</string>
+    <!-- %1$s is the provider name, e.g. "Google" or "Discord". -->
     <string name="login_sso_provider">Sign in with %1$s</string>
     <string name="login_sso_loading">Loading sign-in options…</string>
     <string name="login_sso_retry">Couldn\'t load sign-in options. Tap to retry.</string>