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

lib/services/src/settings.h

@@ -42,6 +42,8 @@ enum class Setting : uint8_t {
     SshPublic,        // string: its public half, the line to put in a server's authorized_keys
     SshFont,          // int: the terminal's font, 0 (smallest) to 4
     LauncherList,     // bool: the Launcher is a list of names instead of a grid of icons (issue #9)
+    Theme,            // int: the colour theme, an index into ui::palette (issue #10)
+    ThemeLight,       // bool: its light side instead of its dark one
     Count
 };