UI: seven themes, each dark and light, chosen in the Settings

Settings > Theme and Settings > Light or dark pick the colours of every
screen (issue #10): roro9stack (the website's, the default), Catppuccin,
Dracula, Nord, ANSI terminal, Gruvbox and Solarized.

- A palette is 14 roles (lib/ui/src/palette.h). theme::kText and the
  others are now references into the one in use, so the Apps are
  unchanged; the main loop applies it when a Setting changes.
- Every colour is one the RGB332 frame buffer holds exactly: the themes'
  own were rounded, and those that fell together or lost contrast were
  picked by hand. test_palette checks exactness and 18 contrast ratios
  for each palette.
- The Sky view's constellation colours come from the theme.
- `theme [0-6] [light|dark]` on the consoles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

src/ui/screen.h

@@ -14,6 +14,8 @@ class Screen {
     // Takes over the screen while a Firmware Update is received or about to restart.
     void renderUpdate(const std::string& title, const std::string& detail, int percent);
     Canvas& canvas() { return canvas_; }
+    // The colours changed: an App that keeps what it drew has to draw it all again.
+    void invalidate() { lost_ = true; }
 
    private:
     Canvas canvas_;