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)

apps/desktop/src/lib/deviceBackup.ts

@@ -91,3 +91,52 @@ export async function importDeviceKey(
     s.memzero(key);
   }
 }
+
+// Full-device backup. Wraps userId + deviceId + privateKey in a JSON payload
+// before encrypting, so a restore flow can re-seed localStorage + vault + server
+// device row without requiring the user to remember IDs.
+export interface DeviceBackupPayload {
+  v: 2;
+  userId: string;
+  deviceId: string;
+  privateKeyB64: string;
+}
+
+export async function exportDeviceBackup(
+  params: {
+    userId: string;
+    deviceId: string;
+    privateKey: Uint8Array;
+    passphrase: string;
+  },
+): Promise<string> {
+  const payload: DeviceBackupPayload = {
+    v: 2,
+    userId: params.userId,
+    deviceId: params.deviceId,
+    privateKeyB64: b64url(params.privateKey),
+  };
+  const bytes = new TextEncoder().encode(JSON.stringify(payload));
+  return exportDeviceKey(bytes, params.passphrase);
+}
+
+export async function importDeviceBackup(
+  backup: string,
+  passphrase: string,
+): Promise<DeviceBackupPayload> {
+  const plain = await importDeviceKey(backup, passphrase);
+  const text = new TextDecoder().decode(plain);
+  try {
+    const obj = JSON.parse(text) as DeviceBackupPayload;
+    if (obj && obj.v === 2 && obj.userId && obj.deviceId && obj.privateKeyB64) {
+      return obj;
+    }
+  } catch {
+    /* fall through */
+  }
+  throw new Error('Backup format not supported — v2 expected');
+}
+
+export function decodePrivateKeyFromBackup(payload: DeviceBackupPayload): Uint8Array {
+  return unb64url(payload.privateKeyB64);
+}