Add channel memory: CH button, dropdown, editor form, smart CTCSS/RPT auto-clear

- New ChannelStore.pas: TChannel record + JSON persistence (channels.json)
- New ChannelsForm.pas: full channel editor (list + fields, pre-fills from current radio state)
- CH button in RX block: left-click opens dropdown, right-click opens editor
- Active channel: button highlights with channel name, deactivates on VFO move
- ApplyChannel: auto-switches XVTR/HF band based on channel frequency; saves old
  band state before switching to prevent cache corruption
- SaveCurrentBand: skip save when VFO > 61 MHz (prevents HF cache corruption
  from VHF channel application without XVTR)
- DeactivateXvtr: save full XVTR state (was only saving LastFreq)
- CTCSS/RPT from channel marked as auto: cleared automatically when tuning away
  from channel frequency; survives band switches via FXvtrSettings auto-flags
- TXvtrEntry: add LastCTCSSAutoActive, LastFMRptAutoActive fields

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

FMRepeater.pas

@@ -0,0 +1,79 @@
+unit FMRepeater;
+
+{ FM repeater offset helpers — constants, default offsets, auto-direction logic. }
+
+{$mode objfpc}{$H+}
+
+interface
+
+const
+  RPT_NONE  = 0;
+  RPT_MINUS = 1;
+  RPT_PLUS  = 2;
+
+  RPT_2M_BEGIN     = 144000000.0;
+  RPT_2M_END       = 148000000.0;
+  RPT_70CM_BEGIN   = 430000000.0;
+  RPT_70CM_END     = 440000000.0;
+  RPT_DEFAULT_2M   =   600000.0;  // 0.600 MHz
+  RPT_DEFAULT_70CM = 7600000.0;   // 7.600 MHz
+
+  // European 2m repeater input segment: auto-enable minus direction
+  RPT_AUTO_MINUS_LO = 145600000.0;  // 145.600 MHz
+  RPT_AUTO_MINUS_HI = 145787500.0;  // 145.7875 MHz
+
+{ Returns default offset (Hz) for the given visible frequency.
+  70 cm band → 7.600 MHz; everything else → 0.600 MHz. }
+function RptDefaultOffsetHz(FreqHz: Double): Double;
+
+{ Returns RPT_MINUS when FreqHz falls in the 2 m auto-minus segment,
+  RPT_NONE otherwise. }
+function RptAutoDir(FreqHz: Double): Integer;
+
+{ Formats OffsetHz as MHz with 3 decimal places, e.g. "0.600". }
+function RptFormatOffset(OffsetHz: Double): string;
+
+{ Parses a MHz string ("0.600", "7.6", "7,600") to Hz.
+  Returns 0 on parse error or out-of-range value. }
+function RptParseOffset(const S: string): Double;
+
+implementation
+
+uses SysUtils;
+
+function RptDefaultOffsetHz(FreqHz: Double): Double;
+begin
+  if (FreqHz >= RPT_70CM_BEGIN) and (FreqHz <= RPT_70CM_END) then
+    Result := RPT_DEFAULT_70CM
+  else
+    Result := RPT_DEFAULT_2M;
+end;
+
+function RptAutoDir(FreqHz: Double): Integer;
+begin
+  if (FreqHz >= RPT_AUTO_MINUS_LO) and (FreqHz <= RPT_AUTO_MINUS_HI) then
+    Result := RPT_MINUS
+  else
+    Result := RPT_NONE;
+end;
+
+function RptFormatOffset(OffsetHz: Double): string;
+begin
+  Result := Format('%.3f', [OffsetHz / 1000000.0]);
+end;
+
+function RptParseOffset(const S: string): Double;
+var
+  V:    Double;
+  Code: Integer;
+  Norm: string;
+begin
+  Norm := StringReplace(Trim(S), ',', '.', [rfReplaceAll]);
+  Val(Norm, V, Code);
+  if (Code = 0) and (V > 0.0) and (V <= 100.0) then
+    Result := V * 1000000.0
+  else
+    Result := 0.0;
+end;
+
+end.