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/App.tsx

@@ -36,7 +36,19 @@ export function App() {
           <FriendshipsProvider>
             <ConversationsProvider>
               <CallProvider>
-                <BrowserRouter>
+                <BrowserRouter
+                  future={{
+                    // Opt into v7 behaviour early so the upgrade is a no-op:
+                    // - `v7_startTransition` wraps navigations in startTransition
+                    //   so Suspense / concurrent rendering deal with the new tree
+                    // - `v7_relativeSplatPath` matches relative paths inside
+                    //   splat routes against the parent splat segment (not the
+                    //   full matched path). Our tree has no splat routes today
+                    //   but this kills the runtime warning and future-proofs.
+                    v7_startTransition: true,
+                    v7_relativeSplatPath: true,
+                  }}
+                >
                   <Routes>
                     <Route element={<RouteBoundary scope="auth" />}>
                       <Route path="/auth" element={<AuthPage />} />