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/wailsjs/go/main/App.d.ts

@@ -86,6 +86,8 @@ export function BulkUpdateField(arg1:Array<number>,arg2:string,arg3:string):Prom
 
 export function BulkUpdateQSL(arg1:Array<number>,arg2:main.QSLBulkUpdate):Promise<number>;
 
+export function CWDecoderRunning():Promise<boolean>;
+
 export function ChatAvailable():Promise<boolean>;
 
 export function CheckForUpdate():Promise<main.UpdateInfo>;
@@ -368,6 +370,8 @@ export function GetCATSettings():Promise<main.CATSettings>;
 
 export function GetCATState():Promise<cat.RigState>;
 
+export function GetCWDecoderPitch():Promise<number>;
+
 export function GetCatalogCodes():Promise<Array<string>>;
 
 export function GetChangelog():Promise<Array<main.ChangelogEntry>>;
@@ -858,6 +862,8 @@ export function SetCATFrequency(arg1:number):Promise<void>;
 
 export function SetCATMode(arg1:string):Promise<void>;
 
+export function SetCWDecoderPitch(arg1:number):Promise<void>;
+
 export function SetClublogCtyEnabled(arg1:boolean):Promise<void>;
 
 export function SetClublogMostWantedEnabled(arg1:boolean):Promise<void>;
@@ -876,8 +882,12 @@ export function SetUIPref(arg1:string,arg2:string):Promise<void>;
 
 export function SetUltrabeamDirection(arg1:number):Promise<void>;
 
+export function StartCWDecoder():Promise<void>;
+
 export function StationSetRelay(arg1:string,arg2:number,arg3:boolean):Promise<void>;
 
+export function StopCWDecoder():Promise<void>;
+
 export function SwitchCATRig(arg1:number):Promise<void>;
 
 export function SyncPOTAHunterLog(arg1:boolean,arg2:boolean):Promise<main.POTASyncResult>;