… Theme.h helper

Pre-consolidation the codebase had eight near-duplicate QSlider stylesheet
constants across the applet family, each with subtle differences:

  TxApplet / PhoneApplet / PhoneCwApplet / RxApplet  cyan handle, no fill
  WaveApplet                                         sub-page + bordered handle
  AetherDspWidget / DspParamPopup                    hover-cyan, distinct groove
  VfoWidget                                          plain light, third groove bg
  SpectrumOverlayMenu                                two more local variants
  StripFinalOutputPanel / StripWaveformPanel         themed amber sub-page
  PanadapterApplet (CW sens)                         partially themed cyan

Operator-visible drift: same slider widget, different look in three
adjacent applets. Maintainability drift: a Phase 4 light-theme tweak to
"slider handle colour" would have to touch nine separate stylesheet
literals.

This commit picks the canonical look:

  Sub-page fill (Wave-style value indicator) +
  Plain light handle (Vfo-style, no border, no hover)

…and exposes it through one helper:

  Theme::primarySliderStyleTemplate(accentToken = "color.accent")
  Theme::applyPrimarySliderStyle(slider, accentToken = "color.accent")

applyPrimarySliderStyle() routes through ThemeManager::applyStyleSheet()
so the slider gets:
  * Token-resolved {{color.background.1}} groove
  * {{color.accent}} (or caller-supplied token) sub-page fill
  * {{color.text.primary}} handle
  * Free live re-theme on theme change via the reverse-map
  * Both horizontal and vertical orientation rules

Variant tokens — the two transmit-adjacent Strip panels pass
"color.accent.warning" for the amber sub-page that signals TX state.
Same shape, different colour token; no per-call-site stylesheet needed.

## Files migrated (13)

  Theme.h                       + helpers, updated default QSlider
                                  rules in appStylesheetTemplate()
  TxApplet / PhoneApplet /      collapse kSliderStyle const,
   PhoneCwApplet / RxApplet     swap setStyleSheet → applyPrimarySliderStyle
   WaveApplet / DspParamPopup /
   VfoWidget / AetherDspWidget /
   SpectrumOverlayMenu /
   PanadapterApplet (CW sens)
  StripFinalOutputPanel /       same migration; amber variant via
   StripWaveformPanel           accentToken parameter

Two slider sites left intentionally untouched:
  * PanadapterApplet Lo/Hi range sliders — deliberate visual
    de-emphasis for secondary range controls; tiny gray handle is
    a load-bearing visual cue, not drift.
  * MeterSlider — already its own widget for VU/level meters with
    its own canonical style (handled in #3106).

## Build verified clean

  AetherSDR (615 ninja steps) + all 320 targets including every test.

73, Jeremy KK7GWY & Claude (AI dev partner)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…der borders

Two follow-ups to #3118 from Jeremy's smoke test:

1. **Pan / balance sliders now fill from the centre outward.**  The L↔R
   pan in RxApplet and the L↔R balance in VfoWidget both use
   CenterMarkSlider, whose neutral position is the midpoint of the
   range.  The default stylesheet sub-page rule paints (0 → handle),
   which reads visually wrong on a centre-anchored control — a value
   slightly left of centre lit up the whole left half of the groove.

   CenterMarkSlider::paintEvent now over-paints the unwanted half of
   the sub-page with the groove colour (effectively erasing it) and
   adds the (centre → handle) accent fill when the handle sits left of
   centre.  Clipping excludes the handle pixel disc so the overpaint
   never bleeds into it.

   The fill resolves through ThemeManager so it tracks theme switches
   for free, same as every other primary slider in the codebase.

2. **All remaining `QSlider` border styling is now explicitly suppressed.**
   The pre-consolidation SpectrumOverlayMenu had `QSlider { border: none; }`
   in its local kSliderStyle to kill the platform-style 1-px border that
   shows up on some themes.  Dropping that line during the #3118 sweep
   re-introduced visible borders on the FFT AVG / FPS / Line / Fill /
   Floor / NB Blank / Black Level / Wtrfall Gain / Wtrfall Rate /
   BG Opacity sliders in the Spectrum Overlay Display tab.

   Theme::primarySliderStyleTemplate() and the QSlider section of
   appStylesheetTemplate() both now include `border: none;` on the
   QSlider element itself + on every subcontrol (groove / sub-page /
   add-page / handle) so the rule wins regardless of which platform
   style is providing the default.

The CenterMarkSlider class is currently duplicated between RxApplet.cpp
and VfoWidget.cpp; both copies got the identical fix. Consolidating
into a single shared header is reasonable cleanup but orthogonal to the
fix — keeping it for a follow-up.

73, Jeremy KK7GWY & Claude (AI dev partner)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>