Add a light theme toggle

[vnmoorthy]

Skylog is dark-only today. A light theme (keeping the orange accent) would widen the audience and is a self-contained task.

Scope:

• New theme slice in src/state/store.ts (persisted)
• Tailwind darkMode: "class" (already set) — toggle the dark class on html
• Light-mode colour overrides for the ink palette in tailwind.config.ts
• Toggle button in TopBar.tsx, keyboard shortcut c
• Make sure MapLibre basemap swaps to CartoDB light_all in light mode

Files to touch: store.ts, TopBar.tsx, LiveMap.tsx, tailwind.config.ts, HelpModal.tsx (shortcut row).

Test with: pnpm typecheck && pnpm test && pnpm build.