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/settings/IntegrationsTab.tsx

@@ -36,6 +36,11 @@ export function IntegrationsTab() {
         icon={<Sparkles size={16} />}
       >
         <div className="settings-card">
+          <div
+            style={{ fontWeight: 700, fontSize: 13, lineHeight: 1.5, marginBottom: 'var(--space-3)', color: 'var(--text-secondary)' }}
+          >
+            {t('settings.discordRichPresenceNotice')}
+          </div>
           <div className="settings-toggle-row">
             <div>
               <div style={{ fontWeight: 500 }}>{t('settings.discordRichPresence')}</div>
@@ -160,6 +165,7 @@ export function IntegrationsTab() {
             <div>
               <div style={{ fontWeight: 500 }}>{t('settings.nowPlayingEnabled')}</div>
               <div style={{ fontSize: 12, color: 'var(--text-muted)' }}>{t('settings.nowPlayingEnabledDesc')}</div>
+              <div style={{ fontSize: 12, fontWeight: 700, color: 'var(--text-secondary)', marginTop: 4 }}>{t('settings.nowPlayingPluginNote')}</div>
             </div>
             <label className="toggle-switch" aria-label={t('settings.nowPlayingEnabled')}>
               <input type="checkbox" checked={auth.nowPlayingEnabled} onChange={e => auth.setNowPlayingEnabled(e.target.checked)} />