perf(P6B.T10): i18next tree-shake — prune dead locale keys

Audited all four i18next namespaces (common, auth, errors, app) against
static t() call-site grep across the entire desktop source tree.
Pruned 40 dead keys per locale, 80 total lines removed across 6 files.

Dead keys removed:
- app: call.{e2ee_active_hint,still_live,voice_connected},
       chats.{show_archived,show_active}, admin.nav,
       friends.confirm_unfriend, settings.{danger_zone,this_device,
       presence,section_ringtone}
- auth: signed_in.{title,session_active,user_id,admin,yes,no,sign_out,
        device_active,device_platform,device_registered_at},
        entire device.* section (old device-registration UI)
- common: loading, cancel, retry, online, offline, idle, dnd, invisible

Dynamic-key patterns were found and respected: t('errors:'+code)
keeps all errors keys; t('app:presence.'+val) keeps all presence keys;
t('app:annotator.tool.'+id) uses defaultValue so its locale entries
were not required.

packages/shared/src/i18n/locales/de/auth.json

@@ -50,28 +50,8 @@
   "footer_studio": "Supabase Studio",
   "legal_note": "Mit der Registrierung akzeptierst du, dass der Server nur Chiffretext sieht.",
   "signed_in": {
-    "title": "Angemeldet",
-    "session_active": "Sitzung aktiv",
-    "user_id": "Benutzer-ID",
     "email": "E-Mail",
     "username": "Benutzername",
-    "display_name": "Anzeigename",
-    "admin": "Admin",
-    "yes": "Ja",
-    "no": "Nein",
-    "sign_out": "Abmelden",
-    "device_active": "Aktives Gerät",
-    "device_platform": "Plattform",
-    "device_registered_at": "Registriert"
-  },
-  "device": {
-    "title": "Dieses Gerät registrieren",
-    "subtitle": "Erzeugt ein X25519-Schlüsselpaar. Der private Schlüssel bleibt auf diesem Gerät.",
-    "name_label": "Gerätename",
-    "name_hint": "Erscheint in deiner Geräteliste. Wähle einen erkennbaren Namen.",
-    "name_placeholder": "Dennis Laptop",
-    "cta": "Gerät registrieren",
-    "cta_loading": "Schlüsselpaar wird erzeugt…",
-    "security_note_dev": "Dev-Build: Privater Schlüssel liegt unverschlüsselt im localStorage. Stronghold folgt vor dem Release."
+    "display_name": "Anzeigename"
   }
 }