feat: device backup/restore + quick wins + username casing

Backup / restore flow:
- deviceBackup.ts: v2 format wrapping userId + deviceId + privateKey in
  an encrypted JSON payload so restore can re-seed localStorage, vault,
  and reattach to the existing server-side device row without provisioning
  a new one (conv-key bundles stay valid, no "awaiting key" state)
- shared/auth: restoreDeviceFromServerRecord — verifies session.user.id
  matches the backup's userId, confirms the server device row still
  exists, then writes the private key into the local secret store
- BackupExportDialog — passphrase + confirm, generates portable string,
  copy + download .txt
- DeviceRestore — textarea + passphrase → seeds vault + writes
  deviceId cache, treats this install as the original device
- DevicePage now has tabs "Neu einrichten" / "Backup wiederherstellen"
- BackupPromptBanner — post-registration nudge, reads sessionStorage
  signal from fresh provisions and persists "never-ask-again" in
  localStorage so it stops nagging
- SettingsPage backup section: uses the new dialog; removes the
  dangerous in-place key import (restore now lives in the device flow)

Username casing:
- Migration 20260420000002 drops lower() from the handle_new_user trigger
  and widens the regex to [A-Za-z0-9_]. profiles.username is citext so
  uniqueness + lookups stay case-insensitive regardless of stored casing
- Shared auth: trim() only, no toLowerCase on signup/lookups/search.
  ilike handles CI anyway and citext makes client normalisation redundant
- AuthPage regex + input preserve case, FriendsPage search preserves case
- i18n (de+en): updated username_hint / username_invalid / ERR_USERNAME_INVALID
  to reflect the new rule

Quick wins:
- React Router v7 future flags (v7_startTransition + v7_relativeSplatPath)
  set on BrowserRouter — silences the upgrade warning
- appUpdates.checkForUpdate: swallow benign network/fetch/"could not
  fetch valid release JSON" cases silently instead of console spam
- osNotify: persist an "asked" marker in localStorage so the permission
  prompt only fires once per install (OS already persists the answer,
  but the plugin re-queries loudly otherwise)

packages/shared/src/auth/device.ts

@@ -152,6 +152,56 @@ export async function saveDevicePrivateKey(
   await secretStore.setSecret(privateKeySecretName(userId, deviceId), privateKey);
 }
 
+// Restores a device record from a backup by re-seeding the local private key
+// store for an EXISTING server-side device row. Does NOT insert a new row —
+// the original row is kept intact so conversation-key bundles stay valid.
+// Throws when the server-side device was removed (the backup is then unusable;
+// user must provision a fresh device and get conv-keys shared from another
+// live device).
+export async function restoreDeviceFromServerRecord(params: {
+  client: AppSupabaseClient;
+  secretStore: SecretStore;
+  userId: string;
+  deviceId: string;
+  privateKey: Uint8Array;
+}): Promise<DeviceRecord> {
+  const { data: session } = await params.client.auth.getUser();
+  if (!session.user) throw new Error('not authenticated');
+  if (session.user.id !== params.userId) {
+    throw new Error(
+      'Backup is for a different account — sign in as the owner before restoring.',
+    );
+  }
+
+  const { data: row, error } = await params.client
+    .from('devices')
+    .select('id, name, platform, public_key, last_seen_at')
+    .eq('id', params.deviceId)
+    .eq('user_id', params.userId)
+    .maybeSingle();
+  if (error) throw error;
+  if (!row) {
+    throw new Error(
+      'Device record not found on server — it was removed. Backup is no longer valid; register a new device instead.',
+    );
+  }
+
+  await saveDevicePrivateKey(
+    params.secretStore,
+    params.userId,
+    params.deviceId,
+    params.privateKey,
+  );
+
+  return {
+    id: row.id,
+    name: row.name,
+    platform: row.platform,
+    publicKey: pgHexToBytes(row.public_key),
+    lastSeenAt: row.last_seen_at,
+  };
+}
+
 // Lightweight helpers for platforms that want to cache their current device id
 // in JSON storage (separate from the secret store, which only holds raw bytes).
 export const DEVICE_ID_STORAGE_KEY_PREFIX = 'chatapp.device_id';