Collect Android browser sign-ins after the app was stopped

The Android app kept a pending browser sign-in in a thread inside the
login screen. Android often kills the process while the browser is in
front, so the approval reached the server but nothing collected it, and
the app came back signed out with nothing saved.

The pending device code is now kept in Prefs and collected whenever the
login screen resumes. The app passes return_url=keeporg://device-approved
when it starts the sign-in, and the server's approval page, after a
password or single sign-on, links back to it. Only the keeporg: scheme is
accepted. Against a server older than 0.9, which rejects the field, the
app retries without it.

Verified on an Android 15 emulator: approval collected after the process
was killed, and the fallback against the 0.8 server.

Server 0.9.0, Android 0.3.0 (versionCode 3).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/oidc_test.go

@@ -140,7 +140,8 @@ func TestOIDCApprovesAWaitingClient(t *testing.T) {
 	idp := newFakeIdP(t, "subject-2", "dev@example.com", "devuser")
 	h := newHarnessWith(t, idp.withOIDC(true, "everyone"))
 
-	status, start := h.call("POST", "/api/v1/auth/device/start", "", map[string]string{"label": "laptop"})
+	status, start := h.call("POST", "/api/v1/auth/device/start", "",
+		map[string]string{"label": "phone", "return_url": "keeporg://device-approved"})
 	if status != http.StatusOK {
 		t.Fatalf("device start: %d %v", status, start)
 	}
@@ -151,6 +152,10 @@ func TestOIDCApprovesAWaitingClient(t *testing.T) {
 	if status != http.StatusOK || !strings.Contains(page, "Device approved") {
 		t.Fatalf("device approval through single sign-on: %d\n%s", status, page)
 	}
+	// A phone is handed back to its app from the provider's redirect too.
+	if !strings.Contains(page, `href="keeporg://device-approved"`) {
+		t.Fatalf("no link back to the app after single sign-on:\n%s", page)
+	}
 	// A device approval must not also hand the browser a session cookie.
 	if b.cookie() != "" {
 		t.Fatal("approving a device set a web session cookie")