Theme pairs with dark mode toggle, gear active state, PALETTE_DEFAULT_DARK (#127)

- Collapse the 10 theme cards into 5 pairs: light swatches top row, dark bottom row
- Dark mode toggle in settings and topbar (sun/moon inline SVG)
- Toggling dark mode switches to the other variant of the selected pair
- Persist pair id in localStorage palette-theme, dark flag in palette-dark
- Head script resolves base+dark to variant; URL ?theme= accepts both ids and wins
- Settings gear highlights like nav tabs on /settings
- PALETTE_DEFAULT_DARK env var sets server default dark state (default on)
- Server-side resolution helpers + tests

cmd/palette/main.go

@@ -42,6 +42,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	web.SetDefaultDark(os.Getenv("PALETTE_DEFAULT_DARK"))
 	srv := api.NewServer(st, cfg, ui, ss, adminKey)
 	log.Printf("palette listening on %s", cfg.Addr)
 	log.Fatal(http.ListenAndServe(cfg.Addr, srv.Routes()))