feat: light theme with runtime Light/Dark toggle

[Fyfar]

Summary

Adds a light theme. The header's ⋮ button didn't do anything; it's now a text toggle that switches themes at runtime, with no page reload.

• The label names the theme a click switches to: Light while dark, Dark while light.
• Only neutral colors change (backgrounds, panels, borders, text, chart axes/grid/tooltip). Green/red (start/stop, pass/fail), the blue accent and amber stay the same in both themes.
• Switching leaves the connection, recording, recorded data, current view and settings untouched.
• The choice persists (localStorage key multimeter-live:theme). An inline pre-paint script in layout.tsx applies it, so a stored light theme never flashes dark. Defaults to dark; the OS color-scheme preference is not followed. If storage is blocked, it falls back to dark and the toggle still works for the session.
• Version 2.0.3 → 2.1.0.

How

• app/globals.css: :root[data-theme="light"] redefines the 6 neutral @theme tokens. Tailwind v4 utilities resolve via var(--color-*), so every component restyles without edits. color-scheme follows the theme, which covers native inputs, selects and scrollbars.
• app/page.tsx: theme state plus toggleTheme; the AppHeader button replaces MoreVertical.
• components/RealtimeChart.tsx: canvas neutrals come from a per-theme CHART_NEUTRALS palette. theme recreates the chart and is also a redraw dependency, so the chart repaints while disconnected.

Testing

• npm run lint, npx tsc --noEmit, npm run check, npm run build, npm run fallow: all pass; fallow reports no new findings.
• Headless Chrome against the static build passed checks for: label/theme flip on click, Enter and Space; no reload; unchanged semantic colors; persistence across reload with the theme set before DOMContentLoaded; storage-blocked fallback; no hydration warnings.
• Hardware (ZOYI ZT703s): checked every view, the no-data warning, toggling during a live recording, and charts with data in both themes.

Known trade-off

Green/amber text on white has low contrast (#22c55e ≈ 2.3:1). It's kept on purpose, so start/stop colors stay identical across themes. The PWA theme_color also stays dark.