Fullscreen player polish + Discord Rich Presence fixes (#1068)

* fix(fullscreen-player): drop track-number prefix from title, stop clipping descenders

The big title showed a zero-padded queue position ("11. ") before the
song name; remove it (the position still lives in the top bar). Also bump
line-height 1.05 -> 1.25 so overflow:hidden no longer crops descenders
(g, j, p, q, y) at 46px.

* feat(settings): clarify built-in Discord RP vs official Navidrome plugin

Add a bold notice atop the Discord Rich Presence block explaining it is
the built-in integration, and that users wanting the official Navidrome
Discord RP plugin should leave it off and enable "Show in Now Playing"
instead. Add a matching bold note to the Now Playing setting. New i18n
keys discordRichPresenceNotice / nowPlayingPluginNote across all 5 locales.

* fix(discord-rp): use HTTPS cover URL instead of local file:// path

Discord Rich Presence images are fetched by Discord's servers, so
large_image must be a key or an https:// URL they can reach. Since the
cover pipeline moved to an on-disk webp cache, coverArtUrlForDiscord
returned a file://.../800.webp path on cache hit, which Discord cannot
load and silently falls back to the app icon. Always hand it the
getCoverArt URL; MPRIS keeps the local file:// path unchanged.

* i18n(settings): fill discordRichPresenceNotice + nowPlayingPluginNote for es, nb, ro, ru

The two new Discord-RP notice keys had only landed in 5 of the 9 locales;
complete the set so no locale falls back to English.

* docs(changelog): fullscreen title + Discord RP fixes (PR #1068)

src/components/fullscreenPlayer/FullscreenPlayerStatic.tsx

@@ -75,10 +75,6 @@ export default function FullscreenPlayerStatic({ onClose }: Props) {
   const [queueOpen, setQueueOpen] = useState(false);
   const [lyricsOpen, setLyricsOpen] = useState(false);
 
-  // Prefix the title with the queue position so it matches "Track x / N".
-  const titlePrefix = queueLen > 0
-    ? `${String(queueIndex + 1).padStart(2, '0')}. `
-    : '';
   const metaParts = useMemo(
     () => [currentTrack?.year?.toString(), currentTrack?.genre].filter(Boolean) as string[],
     [currentTrack?.year, currentTrack?.genre],
@@ -139,7 +135,7 @@ export default function FullscreenPlayerStatic({ onClose }: Props) {
               : <div className="fsp-cover-img fsp-cover-img--empty" />}
           </div>
           <div className="fsp-info-text">
-            <p className="fsp-title">{titlePrefix}{currentTrack?.title ?? '—'}</p>
+            <p className="fsp-title">{currentTrack?.title ?? '—'}</p>
             <p className="fsp-artist">{currentTrack?.artist ?? '—'}</p>
             {currentTrack && (
               <div className="fsp-meta">