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/magic-link.ts

@@ -3,7 +3,8 @@ import type { AppSupabaseClient } from '../supabase/client.js';
 
 export interface SignupParams {
   email: string;
-  // Login handle. Lowercased server-side; must match ^[a-z0-9_]{3,32}$ once lowercased.
+  // Login handle. Stored with original casing but uniqueness is case-insensitive
+  // (citext). Must match ^[A-Za-z0-9_]{3,32}$.
   username: string;
   // Optional human-readable name. Defaults to username server-side if omitted.
   displayName?: string;
@@ -29,7 +30,9 @@ export async function signUpWithMagicLink(
       shouldCreateUser: true,
       data: {
         invite_code: params.inviteCode,
-        username: params.username.toLowerCase(),
+        // Preserve case. `profiles.username` is citext so uniqueness + lookups
+        // stay case-insensitive regardless of stored casing.
+        username: params.username.trim(),
         display_name: params.displayName ?? params.username,
         ...(params.locale ? { locale: params.locale } : {}),
       },