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/app.json

@@ -53,8 +53,6 @@
     "archive": "Archivieren",
     "unarchive": "Entarchivieren",
     "archived_title": "Archiv",
-    "show_archived": "Archiv anzeigen",
-    "show_active": "Aktive anzeigen",
     "archived_empty_title": "Nichts archiviert",
     "archived_empty_subtitle": "Archivierte Unterhaltungen erscheinen hier.",
     "mute": "Stummschalten",
@@ -84,8 +82,6 @@
     "join": "Beitreten",
     "in_call": "Im Anruf",
     "waiting_for_peers": "Warte auf andere…",
-    "voice_connected": "Sprachchat verbunden",
-    "still_live": "Anruf läuft noch",
     "share_screen": "Bildschirm teilen",
     "stop_share_screen": "Screen-Share stoppen",
     "is_sharing_screen": "{{name}} teilt den Bildschirm",
@@ -133,11 +129,9 @@
     "action_unfriend": "Entfernen",
     "action_accept": "Annehmen",
     "action_decline": "Ablehnen",
-    "action_cancel": "Abbrechen",
-    "confirm_unfriend": "Diesen Freund entfernen?"
+    "action_cancel": "Abbrechen"
   },
   "admin": {
-    "nav": "Admin",
     "title": "Admin-Panel",
     "settings_title": "Globale Einstellungen",
     "invites_enabled": "Neue Registrierungen erlauben",
@@ -188,15 +182,11 @@
     "screen_share_quality": "Qualität",
     "screen_share_hint": "WebRTC passt Bitrate + Auflösung dynamisch an die Netzwerkqualität an (SVC/VP9). Die Werte sind Obergrenzen. Änderungen greifen beim nächsten Call.",
     "language": "Sprache",
-    "presence": "Status",
     "show_read_receipts": "Lesebestätigungen anzeigen",
     "show_read_receipts_hint": "Wenn aus, sehen andere nicht wann du ihre Nachrichten gelesen hast — und du siehst nicht wann sie deine gelesen haben.",
     "allow_dms_strangers": "DMs von Fremden erlauben",
     "allow_dms_strangers_hint": "Wenn aus, können dich nur Freunde direkt anschreiben.",
-    "this_device": "Dieses Gerät",
-    "danger_zone": "Gefahrenzone",
     "sign_out": "Abmelden",
-    "section_ringtone": "Klingelton",
     "ringtone_incoming": "Eingehender Anruf",
     "ringtone_default_active": "Standard-Klingelton (Doppelton)",
     "ringtone_custom_active": "{{name}} · {{size}} MB",