feat(ui): reusable app-styled confirm dialog, replacing window.confirm

Add a promise-based ConfirmProvider + useConfirm() hook (built on the Modal shell,
liquid-glass styling, danger variant, Esc/backdrop = cancel, Enter = confirm) so
confirmations match the app instead of the native browser dialog. Wire it in at the
app root and replace both window.confirm call sites (playlist delete, channel
unsubscribe). Trilingual common.confirm/confirmTitle strings.

frontend/src/i18n/locales/en/common.json

@@ -3,6 +3,8 @@
   "termsOfService": "Terms of Service",
   "close": "Close",
   "cancel": "Cancel",
+  "confirm": "Confirm",
+  "confirmTitle": "Are you sure?",
   "save": "Save",
   "loading": "Loading…",
   "language": "Language",