Extract spectrum/waterfall rendering into SpectrumView.pas

- New unit SpectrumView.pas (TSpectrumView): encapsulates all
  spectrum, waterfall, ruler and S-meter rendering; ~900 lines
  removed from MainForm.pas
- Fix marker bug: FMarkerActive/FMarkerX were duplicated in
  MainForm and TSpectrumView — mouse handlers now write directly
  to FSpecView, stale MainForm fields removed
- Fix filter band and ruler not following VFO: add SyncSpecViewFreq
  helper called at every draw site and at top of SpectrumTimerTick
- Remove dead code: WFALL_PALETTE (34 lines), FSplitter, LblAGC,
  LblVol, WinFirewall/IntfGraphics/FPImage from uses
- Fix pre-existing bug: SidetoneFreqLo := 600 overflowed Byte,
  replaced with Hi(600)/Lo(600)

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

ewsdr.lpi

@@ -151,6 +151,10 @@
         <Filename Value="CATTcp.pas"/>
         <IsPartOfProject Value="True"/>
       </Unit>
+      <Unit>
+        <Filename Value="SpectrumView.pas"/>
+        <IsPartOfProject Value="True"/>
+      </Unit>
     </Units>
   </ProjectOptions>
   <CompilerOptions>