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/hu/common.json

@@ -3,6 +3,8 @@
   "termsOfService": "Felhasználási feltételek",
   "close": "Bezárás",
   "cancel": "Mégse",
+  "confirm": "Megerősítés",
+  "confirmTitle": "Biztos vagy benne?",
   "save": "Mentés",
   "loading": "Betöltés…",
   "language": "Nyelv",