feat: crossfade preview suite, loudness normalization, debug overlay

Preview modal audio, five rounds:
- True track-change crossfade: scheduleSettle no longer silences the
  outgoing clip at t=0; preview_load's equal-power fade owns both ramps,
  gated on the incoming voice's first Playing event (audible_marks), with
  failure/overstay fallbacks. CROSSFADE_MS = 340 — locked to the reel's
  card-slide so audio and visuals land together.
- Two-neighbour lease prefetch + a 15-min on-disk clip byte-cache
  (recent_clips): steps in BOTH directions crossfade in ~15 ms; revisits
  never re-download.
- Adaptive settle (isolated steps fire immediately, bursts coalesce) and
  anticipatory auto-advance (step at clip end minus 400 ms) so auto-play
  crossfades over the tail instead of gapping.
- Session resume memory: revisited tracks continue where they left off
  (decoder-level start offset via play_local_from/resume_at); finished
  tracks restart from the top.
- Open/close crossfades with the MAIN player: fade-pause with the
  desired-paused latch landing at ramp end, fade-resume from the paused
  position; MAIN_FADE_GEN guards; focus handshake relaxation documented
  in focusBridge.

Volume normalization (Settings > Playback, default on):
- Dependency-free BS.1770/EBU R128 meter in the player crate (K-weighting,
  400 ms gated blocks, incremental) + analyzer that reads the same growing
  file playback streams from — first plays are measured at download speed.
- Gain layered under user volume; audible start GATED on a preliminary
  measurement (10 s of audio or 800 ms budget) so the level is right from
  the first sample and never changes mid-track; prelim boosts capped 3 dB,
  peak-aware clipping guard, persistent loudness-cache.json.
- Gapless boundaries get their own sample-accurate gain via
  SetNextNormGainDb, fed by preload analysis.

Debug overlay (Ctrl+Shift+D): process-tree CPU / PDH GPU / working-set +
private-commit RAM / network rates / FPS / current-track download progress
read from the backend's own StreamProgress / live normalization state /
preview session phase. Polls only while visible.

131 JS + player + app tests green; every path verified live over CDP.

src-tauri/src/config.rs

@@ -129,6 +129,10 @@ pub struct AppConfig {
     /// Some(0) → unlimited.
     #[serde(default)]
     pub stream_cache_max_mb: Option<u64>,
+    /// Loudness normalization (−14 LUFS target, BS.1770 measurement):
+    /// every track plays at roughly the same perceived level. None → true.
+    #[serde(default)]
+    pub volume_normalization: Option<bool>,
 }
 
 /// Default longest edge for embedded covers when the user hasn't chosen.
@@ -166,6 +170,10 @@ pub fn stream_cache_max_mb(cfg: &AppConfig) -> u64 {
         .unwrap_or(DEFAULT_STREAM_CACHE_MAX_MB)
 }
 
+pub fn volume_normalization(cfg: &AppConfig) -> bool {
+    cfg.volume_normalization.unwrap_or(true)
+}
+
 /// One-time migration after the identifier rename: the app-data dir moved
 /// from `com.trifecta.soulseek-plus` to the new identifier's folder. Rename
 /// the old dir into place so the library DB, config and stream cache