fix(loudness): target sync, effective pre-analysis trim, and queue/settings copy (#333)

* fix(loudness): target sync, -14 pre-analysis ref, queue UI, and reseed

- Front: coalesce loudness refresh by target LUFS; replay-gain IPC dedupe keys
  include norm target and effective pre-attenuation so TGT changes apply.
- Rust: placeholder gain before integrated LUFS uses pivot at -14 LUFS; UI gain
  from effective trim; reseed loudness after delete when waveform cache would skip.
- Pre-analysis: store attenuation relative to -14 LUFS; engine and UI use an
  offset for other targets; migrate legacy absolute values on rehydrate.
- Queue/Settings: Loudness/TGT labels vs value buttons; styles; i18n for help.

* fix(i18n): simplify loudness pre-analysis helper copy

Remove reference-target wording from loudness pre-analysis helper text and keep
only the effective adjustment shown for the current LUFS target in all locales.

src/utils/loudnessPlaceholder.ts

@@ -0,0 +1,15 @@
+/**
+ * Before SQLite has integrated LUFS, match Rust `loudness_gain_placeholder_until_cache`:
+ * pivot at -14 LUFS, `true_peak = 0` (no EBU headroom cap), then add pre-attenuation from settings.
+ */
+const PLACEHOLDER_INTEGRATED_LUFS = -14;
+
+export function loudnessGainPlaceholderUntilCacheDb(
+  targetLufs: number,
+  preAnalysisAttenuationDb: number,
+): number {
+  const pre = Math.min(0, Math.max(-24, preAnalysisAttenuationDb));
+  let pivot = targetLufs - PLACEHOLDER_INTEGRATED_LUFS;
+  pivot = Math.max(-24, Math.min(24, pivot));
+  return Math.max(-24, Math.min(24, pivot + pre));
+}

src/utils/loudnessPreAnalysisSlider.ts

@@ -0,0 +1,21 @@
+/** Pre-analysis level is defined relative to a −14 LUFS target; engine uses an offset for other targets. */
+export const LOUDNESS_PRE_ANALYSIS_REF_TARGET_LUFS = -14 as const;
+
+/**
+ * dB actually applied by the engine (and placeholder math) for the current target.
+ * Example: ref −4.5 dB at −14, at −12 → −2.5 dB.
+ */
+export function effectiveLoudnessPreAnalysisAttenuationDb(
+  storedDbRelativeToRef: number,
+  targetLufs: number,
+): number {
+  const stepped = Math.round(storedDbRelativeToRef * 2) / 2;
+  const effective = stepped + (targetLufs - LOUDNESS_PRE_ANALYSIS_REF_TARGET_LUFS);
+  return Math.max(-24, Math.min(0, effective));
+}
+
+/** Stored [−24, 0] dB, meaning “at −14 LUFS target”. */
+export function clampStoredLoudnessPreAnalysisAttenuationRefDb(v: number): number {
+  const n = Math.round(v * 2) / 2;
+  return Math.max(-24, Math.min(0, n));
+}