Add TUI documentation page with generated SVG screenshots

- New docs page (tools/tui.mdx) covering all 5 RF modes, keyboard
  shortcuts, easter eggs, and splash screen with inline SVG screenshots
- Screenshot generation script using Textual headless Pilot API to
  capture 8 screens in demo mode without hardware
- Fix dark mode toggle: migrate from removed App.dark to App.theme API
  (Textual 7.x breaking change)
- Update social link to Gitea repo, add TUI to sidebar

site/astro.config.mjs

@@ -32,9 +32,9 @@ export default defineConfig({
       plugins: [starlightImageZoom(), starlightLinksValidator()],
       social: [
         {
-          icon: 'github',
-          label: 'GitHub',
-          href: 'https://github.com/placeholder/skywalker-1',
+          icon: 'external',
+          label: 'Git Repository',
+          href: 'https://git.supported.systems/warehack.ing/skywalker-1',
         },
       ],
       customCss: ['./src/styles/custom.css'],
@@ -113,6 +113,7 @@ export default defineConfig({
             { label: 'Firmware Loader', slug: 'tools/firmware-loader' },
             { label: 'Tuning', slug: 'tools/tuning' },
             { label: 'SkyWalker RF Tool', slug: 'tools/skywalker' },
+            { label: 'SkyWalker TUI', slug: 'tools/tui' },
             { label: 'EEPROM Utilities', slug: 'tools/eeprom-utilities' },
             { label: 'Debugging', slug: 'tools/debugging' },
             { label: 'TS Analyzer', slug: 'tools/ts-analyzer' },