ui(settings): restructure Normalization section for clarity and breathing room

The mode picker (Off / ReplayGain / LUFS) used to live in the right-hand
action slot of a settings-toggle-row and the per-mode controls were
stacked into the same parent with footer-style help text. Hard to scan,
visually cramped, and odd compared to the rest of the Settings page.

Refactor:

- Mode picker becomes a full-width segmented row with even-flex buttons,
  using the new .settings-segmented utility.
- Each mode renders its own .settings-norm-block sub-section with a
  subtle accent tint and border so the active configuration reads as
  one coherent group.
- Inside the block, every setting is its own .settings-norm-field
  (control row + per-control help text immediately below). 1.1 rem gap
  between fields, 0.45 rem between row and help — clearly groups
  related text without crowding.
- Sliders no longer max-cap at 200 px and instead flex to fill the row.
- Inactive ghost buttons (Off, ReplayGain, RG mode, LUFS targets) get a
  visible border and a faint surface tint so they read as selectable
  slots in dark themes too.
- LUFS mode gets a dedicated note-box explaining that brief volume drift
  on the very first play of a new track is the analysis pass at work,
  not a bug — subsequent plays use the cached measurement, and queued
  tracks are usually pre-analysed during the previous song.
- "Trim before measurement (dB)" renamed to "Pre-analysis attenuation"
  (and equivalents in 8 locales).
- New i18n keys: normalizationDesc, normalizationOff/ReplayGain/Lufs,
  loudnessTargetLufsDesc, loudnessFirstPlayNote, replayGainPreGainDesc,
  replayGainFallbackDesc.

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

src/locales/en.ts

@@ -852,13 +852,22 @@ export const enTranslation = {
     replayGainTrack: 'Track',
     replayGainAlbum: 'Album',
     replayGainPreGain: 'Pre-Gain (tagged files)',
+    replayGainPreGainDesc: 'Universal boost added on top of every ReplayGain calculation. Use to compensate if your library feels too quiet overall.',
     replayGainFallback: 'Fallback (untagged / radio)',
+    replayGainFallbackDesc: 'Applied to tracks (and radio streams) that have no ReplayGain tags at all, so untagged content does not pop louder than the rest.',
     normalization: 'Normalization',
+    normalizationDesc: 'Even out perceived loudness across tracks, albums and radio.',
+    normalizationOff: 'Off',
+    normalizationReplayGain: 'ReplayGain',
+    normalizationLufs: 'LUFS',
     loudnessTargetLufs: 'Target LUFS',
-    loudnessPreAnalysisAttenuation: 'Trim before measurement (dB)',
+    loudnessTargetLufsDesc: 'Reference loudness all tracks are matched to. Lower numbers = louder output. Streaming services typically sit around -14 LUFS.',
+    loudnessPreAnalysisAttenuation: 'Pre-analysis attenuation',
     loudnessPreAnalysisAttenuationDesc:
       'Extra quieting until loudness for this track is saved. Streaming then uses rough guesses, not a full measurement. 0 dB = off; lower = quieter.',
     loudnessPreAnalysisAttenuationReset: 'Reset to default',
+    loudnessFirstPlayNote:
+      'First play of a brand-new track may briefly drift in volume while it is being measured. The next play uses the cached measurement and is rock-solid. Upcoming queue tracks are usually pre-analysed during the previous song, so this rarely happens in practice.',
     crossfade: 'Crossfade',
     crossfadeDesc: 'Fade between tracks',
     crossfadeSecs: '{{n}} s',