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/apps/gnss_app.cpp

@@ -17,14 +17,14 @@ using namespace gnss;
 
 namespace {
 
-// One colour per constellation, readable in the 8-bit frame buffer.
+// One colour per constellation, from the theme: readable on its background.
 uint16_t colorOf(Constellation c) {
     switch (c) {
         case Constellation::Gps: return theme::kAccent;
-        case Constellation::Glonass: return 0xF8C8;  // red
+        case Constellation::Glonass: return theme::kRed;
         case Constellation::Galileo: return theme::kGood;
-        case Constellation::BeiDou: return 0xFEA0;   // yellow
-        case Constellation::Qzss: return 0xC99F;     // violet
+        case Constellation::BeiDou: return theme::kYellow;
+        case Constellation::Qzss: return theme::kViolet;
         default: return theme::kText;
     }
 }