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/i18n/locales/en/auth.json

@@ -30,8 +30,8 @@
     "email_placeholder": "you@example.com",
     "username": "Username",
     "username_placeholder": "dennis",
-    "username_hint": "You log in with this. Lowercase only.",
-    "username_invalid": "Lowercase a–z, digits, underscore · 3–32 chars.",
+    "username_hint": "You log in with this. Case is preserved but not unique (dennis = Dennis).",
+    "username_invalid": "Letters, digits, or underscore · 3–32 chars.",
     "invite_code": "Invite code",
     "invite_hint": "Required · invite-only access."
   },

packages/shared/src/i18n/locales/en/errors.json

@@ -3,7 +3,7 @@
   "network": "Network error. Check your connection.",
   "ERR_NOT_AUTH": "You are not signed in.",
   "ERR_INVITE_CODE_REQUIRED": "Invite code is required.",
-  "ERR_USERNAME_INVALID": "Username must be lowercase a–z, digits, underscore, 3–32 chars.",
+  "ERR_USERNAME_INVALID": "Username must be letters (A–Z, a–z), digits, or underscore, 3–32 chars.",
   "ERR_INVITES_DISABLED": "Signups are currently disabled by the admin.",
   "ERR_INVITE_NOT_FOUND": "Invite code not found.",
   "ERR_INVITE_DISABLED": "This invite has been disabled.",