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/en/auth.json

@@ -50,28 +50,8 @@
   "footer_studio": "Supabase Studio",
   "legal_note": "By signing up you accept that the server sees only ciphertext.",
   "signed_in": {
-    "title": "Signed in",
-    "session_active": "Session active",
-    "user_id": "User ID",
     "email": "Email",
     "username": "Username",
-    "display_name": "Display name",
-    "admin": "Admin",
-    "yes": "yes",
-    "no": "no",
-    "sign_out": "Sign out",
-    "device_active": "Active device",
-    "device_platform": "Platform",
-    "device_registered_at": "Registered"
-  },
-  "device": {
-    "title": "Register this device",
-    "subtitle": "Generates an X25519 keypair. Private key stays on this device.",
-    "name_label": "Device name",
-    "name_hint": "Shown to you in your device list. Keep it recognisable.",
-    "name_placeholder": "Dennis Laptop",
-    "cta": "Register device",
-    "cta_loading": "Generating keypair…",
-    "security_note_dev": "Dev build: private key stored unencrypted in localStorage. Stronghold comes before release."
+    "display_name": "Display name"
   }
 }