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

tui/src/skywalker_tui/app.py

@@ -134,8 +134,10 @@ class SkyWalkerApp(App):
                 self.action_rf_mode(mode)
 
     def action_toggle_dark(self) -> None:
-        self.dark = not self.dark
-        if self.dark:
+        self.theme = (
+            "textual-dark" if self.theme == "textual-light" else "textual-light"
+        )
+        if self.current_theme.dark:
             self.notify(
                 "Welcome to the Dark Side.",
                 title="The Force is strong with this one",