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/ui/src/palette.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <cstdint>
+
+// The colour themes (issue #10): each a dark and a light set of the same roles, as RGB565.
+namespace roro::ui {
+
+struct Palette {
+    uint16_t background, text;
+    uint16_t muted;     // secondary text
+    uint16_t faint;     // grid lines, what stays behind
+    uint16_t bar;       // the Status Bar's background
+    uint16_t barMuted;  // what is idle, on the bar
+    uint16_t accent;    // what is chosen, what is live: also the fill behind a selected row
+    uint16_t onAccent;  // text on an accent, warning or message fill
+    uint16_t warning;
+    uint16_t message;   // someone wrote
+    uint16_t good;      // a Fix, the best of a set
+    uint16_t red, yellow, violet;  // to tell things apart (the constellations of the Sky view)
+};
+
+constexpr int kThemeCount = 7;
+const char* themeName(int theme);                 // "roro9stack", "Catppuccin", "Dracula", ...
+const Palette& palette(int theme, bool light);    // out of range: the first
+
+// What the 8-bit frame buffer shows for a colour, as 0xRRGGBB.
+uint32_t rgb888(uint16_t rgb565);
+
+}  // namespace roro::ui