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/osNotify.ts

@@ -7,10 +7,31 @@ import {
 import { isTauriRuntime } from './globalShortcut';
 
 // Tracks whether permission has already been requested this session so we
-// don't spam the OS prompt. Actual permission state lives in the OS.
+// don't spam the OS prompt. Actual permission state lives in the OS, but we
+// also persist a "we've asked" marker in localStorage so reloads don't
+// re-request (OS would block anyway after denial, but calling it every reload
+// triggers noisy plugin warnings on some platforms).
 let permissionChecked = false;
 let permissionGranted = false;
 
+const ASKED_KEY = 'chatapp.notif.asked';
+
+function readAskedMarker(): boolean {
+  try {
+    return window.localStorage.getItem(ASKED_KEY) === '1';
+  } catch {
+    return false;
+  }
+}
+
+function writeAskedMarker(): void {
+  try {
+    window.localStorage.setItem(ASKED_KEY, '1');
+  } catch {
+    /* quota / private mode */
+  }
+}
+
 export async function ensureNotificationPermission(): Promise<boolean> {
   if (permissionChecked) return permissionGranted;
   permissionChecked = true;
@@ -21,9 +42,13 @@ export async function ensureNotificationPermission(): Promise<boolean> {
   }
   try {
     let granted = await isPermissionGranted();
-    if (!granted) {
+    if (!granted && !readAskedMarker()) {
+      // First-install: prompt the user once. After this we remember via the
+      // marker and never re-prompt — the user can re-enable later via OS
+      // system settings if they change their mind.
       const result = await requestPermission();
       granted = result === 'granted';
+      writeAskedMarker();
     }
     permissionGranted = granted;
   } catch (err: unknown) {