feat(playlists): make YouTube-mirrored playlists editable + syncable

Mirrors (source='youtube') were read-only; per the original two-way design they can
now be edited locally (add/remove/reorder/rename) and pushed back. The read-sync
skips a dirty mirror so it won't clobber unpushed edits; a successful push clears
dirty and lets the mirror refresh. _mark_dirty now covers any YouTube-linked list
(exported local or mirror), and rename marks dirty too. push/delete no longer reject
mirrors. Push reconciles the title as well (cheap playlists.list compare, then
playlists.update only if changed) so a local rename doesn't revert on the next pull.
Frontend: editable = not-built-in (was also excluding mirrors); rows/reorder enabled
for all owned lists; AddToPlaylist popover lists mirrors too (with a YT marker); the
origin chip now reads 'edits sync back'. Trilingual.

frontend/src/i18n/locales/de/playlists.json

@@ -4,7 +4,7 @@
   "syncYoutube": "Von YouTube synchronisieren",
   "syncedToast": "{{count}} Wiedergabelisten von YouTube synchronisiert",
   "syncFailed": "Synchronisierung von YouTube fehlgeschlagen",
-  "ytReadonly": "Von YouTube synchronisiert — vorerst schreibgeschützt",
+  "ytReadonly": "Von YouTube synchronisiert — Änderungen werden zurücksynchronisiert",
   "noneYet": "Noch keine Wiedergabelisten",
   "newPlaylist": "Neue Liste…",
   "itemCount_one": "{{count}} Video",

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

@@ -4,7 +4,7 @@
   "syncYoutube": "Sync from YouTube",
   "syncedToast": "Synced {{count}} playlists from YouTube",
   "syncFailed": "Couldn't sync from YouTube",
-  "ytReadonly": "Synced from YouTube — read-only for now",
+  "ytReadonly": "Synced from YouTube — edits sync back",
   "noneYet": "No playlists yet",
   "newPlaylist": "New playlist…",
   "itemCount_one": "{{count}} video",

frontend/src/i18n/locales/hu/playlists.json

@@ -4,7 +4,7 @@
   "syncYoutube": "Szinkron YouTube-ról",
   "syncedToast": "{{count}} lista szinkronizálva a YouTube-ról",
   "syncFailed": "Nem sikerült a YouTube-szinkron",
-  "ytReadonly": "YouTube-ról szinkronizálva — egyelőre csak olvasható",
+  "ytReadonly": "YouTube-ról szinkronizálva — a módosítások visszaszinkronizálhatók",
   "noneYet": "Még nincs lejátszási lista",
   "newPlaylist": "Új lista…",
   "itemCount_one": "{{count}} videó",