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/appUpdates.ts

@@ -44,10 +44,22 @@ export async function checkForUpdate(): Promise<UpdateState> {
       error: null,
     };
   } catch (err: unknown) {
-    console.warn('checkForUpdate failed', err);
+    // Swallow "no release on GitHub yet" / network-unreachable cases silently.
+    // The updater endpoint serves `latest.json` from GitHub releases; a fresh
+    // repo or offline machine produces a generic "Could not fetch a valid
+    // release JSON" error that has no actionable information for the user —
+    // logging it on every launch just pollutes the console.
+    const msg = err instanceof Error ? err.message : String(err);
+    const benign =
+      /could not fetch a valid release json|network|timed? out|failed to fetch|connection/i.test(
+        msg,
+      );
+    if (!benign) {
+      console.warn('checkForUpdate failed', err);
+    }
     return {
       ...IDLE_UPDATE_STATE,
-      error: err instanceof Error ? err.message : 'update check failed',
+      error: benign ? null : msg,
     };
   }
 }