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/common.json

@@ -1,16 +1,8 @@
 {
   "app_name": "ChatApp",
-  "loading": "Lädt…",
   "finalising_session": "Sitzung wird abgeschlossen…",
-  "cancel": "Abbrechen",
   "save": "Speichern",
   "close": "Schließen",
-  "retry": "Wiederholen",
-  "online": "Online",
-  "offline": "Offline",
-  "idle": "Abwesend",
-  "dnd": "Nicht stören",
-  "invisible": "Unsichtbar",
   "local_stack_online": "Lokaler Stack online",
   "dev_build": "Dev-Build"
 }