feat(playlists): suggestion-row preview UX (#365)

* feat(playlists): suggestion-row preview UX (30s preview, double-click play-next, scroll keep)

Reworks the interaction on the suggestion rows below a playlist so users
can audition songs before deciding what to do with them.

- New "Preview" pill button left of each track title plays a 30-second
  mid-song sample via a parallel HTML5 audio element. The main player
  pauses on the first preview and auto-resumes when the preview ends.
  Switching previews chains without re-resuming. External main-player
  playback (spacebar, mediakey) cancels the preview without resuming.
  Animated underline shows the 30 s progress.
- Double-click the row to insert the song at queueIndex + 1 and skip to
  it ("Play next"). Single-click is intentionally inert so a stray click
  next to the preview button no longer drops a song into the queue by
  accident. Tooltip on the row makes the affordance discoverable.
- The + button still adds to the playlist, and now restores the
  .main-content scroll position so the page no longer jumps to the top
  after pressing it.
- i18n keys in all 8 locales: playlists.preview / previewStop / previewShort
  / previewStopShort / suggestionDoubleClickPlayNext.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(playlists): WebKitGTK NULL-instance crash on preview teardown

cucadmuh hit GLib-GObject-CRITICAL: invalid (NULL) pointer instance /
g_signal_connect_data assertion failed, with the UI freezing after
clicking a preview button. Root cause was the audio-element lifecycle:

- `audio.src = ''` to "stop" leaves WebKitGTK's GStreamer playbin in a
  half-initialized state. The next signal_connect dereferences NULL.
- Creating `new Audio()` per click stacked half-torn-down playbins
  during rapid switches.
- `loadedmetadata` listeners on orphaned audio elements could call
  play() on an already-discarded instance.

Fix: reuse one <audio> element per component, tear down the source
via removeAttribute('src') + load() (which resets the playbin
cleanly), and gate async listeners behind a session counter so stale
metadata events on switched-away previews are ignored.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(playlists): icon-based action buttons + add-on-doubleclick + hint

- Replace text-pill Preview button with circular icon button + animated
  SVG progress ring (Play/Square icon swap when active).
- New Play-next button (filled accent circle, white triangle) sits
  left of the Preview button — explicit affordance for the action that
  used to be a hidden double-click.
- Double-click on a suggestion row now triggers Add-to-playlist (the
  same action as the + button on the right) — discoverable shortcut
  for mouse users.
- Subtitle under the Suggested Songs header announces the add-on-
  doubleclick affordance, in all 8 locales.
- Drop the now-obsolete previewShort / previewStopShort short-label
  keys (text replaced by icons); rename suggestionDoubleClickPlayNext
  → playNextSuggestion to match its new role on the Play-next button.
- Keep the session-counter guard from the prior commit so async
  loadedmetadata handlers from a switched-away preview can't play()
  on a discarded element.

Note: header column labels visually drift from the data columns when
suggestion rows have the action buttons; left as-is for now.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(playlists): apply LUFS pre-analysis attenuation to suggestion previews

When the main player is on loudness normalization, analysed tracks come
out reduced toward target while the unanalysed preview <audio> blasts
the file at its natural level — cucadmuh reported the previews are
audibly louder than the playlist playback.

Apply the user's stored pre-analysis attenuation (the slider value, not
the target-offset effective form) as a linear gain on the preview's
audio.volume. Default −4.5 dB lands the preview at ~60% of the player
volume, which roughly tracks how aggressively the Rust engine pulls
naturally-loud tracks toward target.

If the engine is off, behavior is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* style(playlists): use chevron icon for suggestion preview button

Distinguishes the preview button from the adjacent play-next button
by mirroring the Play Next chevron from the context menu.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/de.ts

@@ -1287,6 +1287,10 @@ export const deTranslation = {
     titleBadge: 'Playlist',
     refreshSuggestions: 'Neue Vorschläge',
     addSong: 'Zur Playlist hinzufügen',
+    preview: 'Vorschau (30 s)',
+    previewStop: 'Vorschau stoppen',
+    playNextSuggestion: 'Als Nächstes spielen',
+    suggestionsHint: 'Doppelklick auf eine Zeile fügt sie zur Playlist hinzu',
     addSelected: 'Ausgewählte hinzufügen',
     cacheOffline: 'Playlist offline speichern',
     offlineCached: 'Playlist gecacht',