feat(themes): add Cyberpunk theme with custom fonts

Add a dark, neon-soaked, dystopian command center theme:

Map colors:
- Near-black base (#0a0a14) with blue-purple undertones
- Magenta motorways (#ff2d6b), purple primary roads (#8833aa)
- Inky dark water (#06061a), barely-visible dark teal vegetation
- Cool white labels with dark halos for readability
- Neon POI icons (cyan, magenta, green)

UI styling:
- Dark translucent panels with magenta accent borders
- Custom fonts: Orbitron (headings), Share Tech Mono (body)
- Entire UI renders in monospace terminal aesthetic
- Magenta shadows, cyan route lines, neon status colors
- Pin markers in magenta (origin) and cyan (destination)

Overlay adjustments:
- Dramatic hillshade with darker shadows
- Very subtle contours (opacityMod: 0.5)
- USFS/BLM trails in purple/magenta/cyan family
- Public lands with muted teal fills

Satellite adjustments:
- Darkened (brightnessMax: 0.30), desaturated, purple-shifted

Also adds --font-heading CSS variable to all themes for future
custom heading font support.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/themes/clean.js

@@ -165,6 +165,7 @@ const cleanUI = {
   // Fonts
   '--font-sans': "'Inter', system-ui, -apple-system, sans-serif",
   '--font-mono': "'JetBrains Mono', ui-monospace, monospace",
+  '--font-heading': "'Inter', system-ui, -apple-system, sans-serif",
   // Backgrounds
   '--bg-base': '#f5f5f5',
   '--bg-raised': '#ffffff',