feat: normalize preview clips to -14 LUFS to match the library

Deezer preview clips are cut from the raw master (not pre-normalized), so
they played several dB hotter than the user's normalized -14 LUFS files —
measured across one artist the previews ranged -0.1 to -8.8 dB from target,
an 8.7 dB spread, so a flat offset would have been wrong either way.

Each clip is now measured with the same BS.1770 meter the main player uses
(preview.rs measure_clip_gain -> loudness::normalization_gain_db) right after
it is written in preview_resolve -- off the async worker, and fully off the
playback path on a prefetched neighbour. The linear gain is stored in the
lease and the reuse cache, and rides UNDER the app-volume mirror per voice
(effective = target_vol * clip_gain), so it composes with the slider, mute
and the crossfade (each clip crosses in at its own matched level). Gated on
the volume_normalization setting; reset on dispose. Debug overlay PREVIEW row
now shows the active clip's norm dB.

Verified live: per-clip gains -0.1..-8.8 dB applied; mute still zeroes the
preview; 39 app + 12 player tests pass.

src/components/DebugOverlay.svelte

@@ -125,6 +125,7 @@
                         {previewUi.snapshot?.phase ?? "—"} · gen {previewUi.snapshot?.requestGeneration ?? "—"} ·
                         idx {previewUi.snapshot?.activeManifestIndex ?? "—"}
                         {#if stats.previewVolume != null}· vol {stats.previewVolume.toFixed(2)}{/if}
+                        {#if stats.previewNormDb != null}· norm {stats.previewNormDb >= 0 ? "+" : ""}{stats.previewNormDb.toFixed(1)} dB{/if}
                     </div>
                 {/if}
             </div>