Summary

Adds Light + System palette variants alongside the historical Dark default, plus a Settings → Appearance picker that persists via `localStorage`.

• `tokens.css` gains `[data-theme="light"]` overrides and a `@media (prefers-color-scheme: light)` block for `[data-theme="system"]` — every component already pulls from the same vars so the swap is global.
• `base.html` injects a tiny inline `<script>` in `` before stylesheets to set `documentElement.dataset.theme` from localStorage. No FOUC.
• `settings.html` grows an Appearance section: three pill cards in an Alpine radiogroup. Choosing one writes `localStorage` and updates `documentElement.dataset.theme` immediately; no server round trip.
• New `internal/panel/assets/css/components/settings.css` with `.settings-card` + `.theme-options` chrome.

Closes #225.

Test plan

• `just build && ./bin/prosa-panel` with `PROSA_PANEL_DEV_LOGIN=1`.
• Open `/settings` — three theme options visible; Dark is highlighted (default).
• Click Light → palette inverts immediately across every page.
• Reload → Light persists; no flash of dark on first paint.
• Click System; toggle macOS Appearance to Light → panel follows.
• `localStorage.removeItem("prosa-theme")` + reload → falls back to Dark cleanly.

🤖 Generated with Claude Code

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}