feat: ClubLog Most Wanted rank in the entry band matrix

Opt-in (Settings → General). Shows a "MW #rank" pill next to the DXCC entity
name in the entry-strip band/slot matrix — ClubLog's Most Wanted ranking
(1 = most wanted), personalised to the operator's callsign and refreshed daily.

- internal/clublog/mostwanted.go: fetch mostwanted.php?api=1&callsign=<CALL>
  (rank→DXCC JSON, real User-Agent), invert to dxcc→rank, cache to
  <dataDir>/clublog_mostwanted.json; NeedsRefresh invalidates on callsign change.
- app.go: keyClublogMostWanted setting, a.clublogMW, startup refresh goroutine,
  activeCallsign() helper, and Get/Set/Download bindings mirroring the cty ones.
  WorkedBefore now carries MWRank (qso.WorkedBefore.MWRank) when enabled.
- BandSlotGrid.tsx: MW pill next to the entity name, colour-tiered by rank.
- SettingsModal General: toggle + download button + status, mirroring the
  ClubLog cty-exceptions block.

Also reorganises the changelog: the theme-persistence fix (landed after the
v0.20.11 tag) plus this feature go under a new 0.20.12 entry; 0.20.11 keeps only
what shipped in its binary.

internal/qso/qso.go

@@ -1370,6 +1370,10 @@ type WorkedBefore struct {
 	DXCCModes     []string   `json:"dxcc_modes"`
 	DXCCBandModes []BandMode `json:"dxcc_band_modes"`
 
+	// MWRank is the ClubLog "Most Wanted" rank for this entity (1 = most wanted),
+	// 0 when unknown or the feature is off. Populated by the app layer, not here.
+	MWRank int `json:"mw_rank,omitempty"`
+
 	// Status grid driving the band×class matrix in the UI. One entry per
 	// (band, class) where ANY QSO exists in this DXCC. Only the highest
 	// status for that cell is kept (call_c > call_w > dxcc_c > dxcc_w).