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/components/AddToPlaylist.tsx

@@ -2,7 +2,7 @@ import { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
-import { Check, ListPlus, Plus } from "lucide-react";
+import { Check, ListPlus, Plus, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
 
 // A small popover (portaled to body, so the feed grid can't clip it) for toggling a
@@ -118,9 +118,9 @@ export default function AddToPlaylist({
     }
   }
 
-  // YouTube-mirrored playlists are read-only until the write-back phase, so they can't be
-  // added to here yet.
-  const lists = (membership.data ?? []).filter((pl) => pl.source !== "youtube");
+  // All of the user's playlists are add targets, including YouTube-linked ones — adding
+  // marks them dirty, and a later "Sync to YouTube" pushes the change back.
+  const lists = membership.data ?? [];
 
   return (
     <>
@@ -167,9 +167,12 @@ export default function AddToPlaylist({
                   >
                     {pl.has_video && <Check className="w-3 h-3" />}
                   </span>
-                  <span className="truncate">
+                  <span className="truncate flex-1 text-left">
                     {pl.kind === "watch_later" ? t("playlists.watchLater") : pl.name}
                   </span>
+                  {(pl.source === "youtube" || pl.yt_playlist_id) && (
+                    <Youtube className="w-3 h-3 shrink-0 text-muted" />
+                  )}
                 </button>
               ))}
             </div>

frontend/src/components/Playlists.tsx

@@ -161,9 +161,10 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
   const plName = (p: { kind: string; name: string }) =>
     p.kind === "watch_later" ? t("playlists.watchLater") : p.name;
   const builtin = detail?.kind === "watch_later";
-  // YouTube-mirrored playlists are read-only here until the write-back phase.
+  // YouTube-mirrored playlists can be edited locally and synced back (the read-sync skips
+  // a dirty mirror so it won't clobber unpushed edits); the chip just marks their origin.
   const mirrored = detail?.source === "youtube";
-  const editable = !builtin && !mirrored;
+  const editable = !builtin;
   const linked = editable && !!detail?.yt_playlist_id;
   const [syncing, setSyncing] = useState(false);
   const [pushing, setPushing] = useState(false);
@@ -253,7 +254,7 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
 
   async function onDragEnd(e: DragEndEvent) {
     const { active: a, over } = e;
-    if (!over || a.id === over.id || selectedId == null || mirrored) return;
+    if (!over || a.id === over.id || selectedId == null) return;
     const oldIndex = items.findIndex((v) => v.id === a.id);
     const newIndex = items.findIndex((v) => v.id === over.id);
     if (oldIndex < 0 || newIndex < 0) return;
@@ -513,7 +514,6 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
                         key={v.id}
                         video={v}
                         index={i}
-                        readOnly={mirrored}
                         onPlay={() => setPlayingIndex(i)}
                         onRemove={() => removeItem(v.id)}
                       />

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",