Introduces a new SliceColorManager singleton (src/gui/SliceColorManager.h/.cpp)
that sits between the static kSliceColors[] defaults and every widget that
renders slice markers, filter bands, or letter badges.  All consumers now call
SliceColorManager::instance().activeColor() / dimColor() / hexActive() instead
of indexing kSliceColors[] directly, so a single colorsChanged() signal
propagates color changes to the entire UI in real time.

Settings are persisted in AppSettings:
  - SliceColorsUseCustom  (True / False)
  - SliceColor0 … SliceColor7  (hex color strings, e.g. "#00d4ff")

Dim colors for custom slots are derived automatically at ~40 % luminance of
the active color so the inactive-slice shading stays readable without requiring
a separate picker.

A new "UI Enhancements" tab is added to the Radio Setup dialog (deferred-load,
same pattern as all other tabs) containing:
  - "Use Aether defaults" / "Custom colors" radio buttons
  - Eight color-swatch buttons (A–H) that open QColorDialog on click; the
    button background reflects the current color so the palette is visible at
    a glance
  - "Reset All to Defaults" button
  - colorsChanged() feedback loop keeps the swatches in sync if another widget
    were ever to trigger a color change

Widgets wired for real-time refresh:
  - SpectrumWidget  — markOverlayDirty() on colorsChanged()
  - VfoWidget       — syncFromSlice() + update() on colorsChanged()
  - RxApplet        — slice selector buttons and header badge
  - CatControlApplet — slice letter badges and TCP-client status labels

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

…ious mutable

  All six public methods (activeColor, dimColor, hexActive, customColor,
  setCustomColor, resetToDefault) previously used sliceId % kSliceColorCount
  directly. In C++, the result of % on a negative left operand is
  implementation-defined-negative, making m_customColors[idx] or m_hexCache[idx]
  undefined behaviour for any negative sliceId. While SliceModel::m_id is
  initialized to 0 and driven from the radio protocol, the public API gave no
  protection against misuse.

  Fix: route all public methods through a private safeIdx() helper that
  Q_ASSERTs sliceId >= 0 in debug builds (catching misuse early) and falls
  back to std::abs(sliceId) % kSliceColorCount in release builds.

  Also remove the mutable qualifier from m_hexCache. It was never needed —
  m_hexCache is only written in non-const rebuildHexCache(), so mutable
  served no purpose and obscured intent.

  The private defaultActive()/defaultDim() helpers are unaffected; they
  receive loop-controlled indices (0–7), not user-supplied slice IDs.

…as the foundation for broader theme customization rather than a narrowly scoped feature, leaving room for future color choosers and visual preference controls.

Two minor cleanups from PR review:

1. Drop the now-dead `#include "SliceColors.h"` from VfoWidget.cpp,
   RxApplet.cpp, and CatControlApplet.cpp. After the SliceColorManager
   conversion these files no longer reference kSliceColors[],
   kSliceColorCount, or SliceColorEntry directly. SpectrumWidget.cpp
   keeps its include because it still uses kSliceColorCount for
   slice→letter mapping (A through H).

2. Drop the contradictory `Q_ASSERT(sliceId >= 0)` from safeIdx().
   The function name promises safety; the assert + std::abs
   combination was self-contradictory (assert fires in debug,
   abs handles it in release). Keeping just std::abs makes the
   defense unconditional and matches what the function is named
   for. Negative slice IDs occur transiently during slice teardown
   (-1 sentinel), so the safety isn't theoretical.

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