feat: CW decoder v2 — rebuilt decode engine, back in the UI

The first RX-audio CW decoder decoded poorly on real signals (vs SDC) and was
removed in cafade0. This rebuilds the DSP core from scratch in
internal/cwdecode, keeping v1's good ideas (pitch lock, Flex cw_pitch
targeting, tiered acquisition) and fixing its proven failures:

- Two-way DEBOUNCE (pending-commit ~0.3 dit): v1 rejected short marks but not
  short spaces, so a one-hop fade inside a dah shattered it into dits — the
  single worst real-signal failure.
- Per-character BATCH dit/dah classification at flush time, using the batch's
  own bimodal boundary — the first letter of an over decodes at any speed; the
  timing clusters (muDit/muDah) are updated with the same attribution, killing
  the classify-then-learn feedback spiral ("all dits at 60 WPM").
- dB-domain envelope with separate floor/peak trackers, hysteresis slicer,
  span CAP (30 dB — else quiet backgrounds put the off-threshold in the
  analysis-window skirts and letters merge) and window de-bias on durations.
- Double squelch: span >= 6 dB AND peak >= bank-median + 9.5 dB (span alone
  cannot reject pure noise).
- Hamming-windowed Goertzel, 16 ms window / 5 ms hop (a 20 ms window left
  40 WPM inter-element gaps with no envelope dip).
- Hardened acquisition (overlapping windows made "3 stable hops" meaningless)
  plus SUPERVISED RE-LOCK: a clearly stronger tone on another pitch sustained
  ~1 s takes over — heals noise locks and follows a QSY.
- Gap-fed dit-length tracking (inter-element gaps are timing evidence too).

Proven by a synthetic-signal test suite (all passing): clean 12-40 WPM, three
pitches, added noise, QSB fading 0.35-1.0, 10 ms dropouts inside dahs, QRM in
auto and targeted modes, noise-only squelch, mid-over speed change 25->15 WPM,
and jittered hand keying (+/-20-25%, 3 seeds).

Wiring and UI restored from git (app_cw.go, Ear header button, decoded-text
strip with WPM/pitch/level + pitch lock + click-a-word-to-fill-callsign, Tools
menu entry) — strip strings translated (cwd.* i18n keys, EN/FR) this time.

frontend/src/lib/i18n.tsx

@@ -32,6 +32,12 @@ const en: Dict = {
   'view.refresh': 'Refresh', 'view.clearFilters': 'Clear filters',
   'tools.qslManager': 'QSL Manager…', 'tools.qslDesigner': 'QSL Card Designer…',
   'tools.winkeyer': 'WinKeyer CW keyer', 'tools.dvk': 'Digital Voice Keyer', 'tools.cwDecoder': 'CW decoder (RX audio)',
+  'cwd.listening': 'listening…', 'cwd.clear': 'Clear', 'cwd.useCall': 'Use as callsign',
+  'cwd.pitchTip': "Lock the decoder to this pitch (Hz). Blank = follow the radio's CW pitch / auto-search.",
+  'cwd.levelTip': 'Audio level {pct}%',
+  'cwd.tipOnCw': 'CW decoder — on (decoding) · click to disable',
+  'cwd.tipOnIdle': 'CW decoder — on, idle until CW mode · click to disable',
+  'cwd.tipOff': 'CW decoder · click to enable (decodes RX audio in CW mode)',
   'tools.net': 'NET Control', 'tools.alerts': 'Alert management…', 'tools.contest': 'Contest mode',
   'alert.tuneHint': 'Click to tune the rig to this spot (freq + mode) and fill the call', 'alert.dismiss': 'Dismiss', 'alert.pending': '{n} recent spot alert(s) — click to view', 'alert.noneShort': 'No recent alerts', 'alert.recent': 'Recent alerts', 'alert.clear': 'Clear',
   'menu.help': 'Help', 'help.about': 'About OpsLog', 'tools.duplicates': 'Find duplicates…',
@@ -366,6 +372,12 @@ const fr: Dict = {
   'view.refresh': 'Rafraîchir', 'view.clearFilters': 'Effacer les filtres',
   'tools.qslManager': 'Gestionnaire QSL…', 'tools.qslDesigner': 'Créateur de carte QSL…',
   'tools.winkeyer': 'Manipulateur CW WinKeyer', 'tools.dvk': 'Manipulateur vocal numérique', 'tools.cwDecoder': 'Décodeur CW (audio RX)',
+  'cwd.listening': 'écoute…', 'cwd.clear': 'Effacer', 'cwd.useCall': "Utiliser comme indicatif",
+  'cwd.pitchTip': "Verrouille le décodeur sur cette tonalité (Hz). Vide = suit le pitch CW de la radio / recherche auto.",
+  'cwd.levelTip': 'Niveau audio {pct}%',
+  'cwd.tipOnCw': 'Décodeur CW — actif (décode) · clic pour désactiver',
+  'cwd.tipOnIdle': 'Décodeur CW — actif, en veille hors mode CW · clic pour désactiver',
+  'cwd.tipOff': 'Décodeur CW · clic pour activer (décode l’audio RX en mode CW)',
   'tools.net': 'Contrôle de NET', 'tools.alerts': 'Gestion des alertes…', 'tools.contest': 'Mode contest',
   'alert.tuneHint': 'Cliquer pour accorder la radio sur ce spot (fréq + mode) et remplir l\'indicatif', 'alert.dismiss': 'Fermer', 'alert.pending': '{n} alerte(s) de spot récente(s) — cliquer pour voir', 'alert.noneShort': 'Aucune alerte récente', 'alert.recent': 'Alertes récentes', 'alert.clear': 'Effacer',
   'menu.help': 'Aide', 'help.about': 'À propos d\'OpsLog', 'tools.duplicates': 'Trouver les doublons…',