feat: add dark mode toggle

[magnus-madsen]

Summary

• Adds a dark mode toggle button (moon/sun icon) next to the "Shareable Link" button in the menu bar
• Uses CSS custom properties for all colors with [data-theme="dark"] overrides
• Persists theme preference to localStorage; auto-detects system prefers-color-scheme on first visit
• Inline script in index.html prevents flash of wrong theme on load
• Ace editor switches between xcode (light) and tomorrow_night (dark) themes
• Bootstrap btn-light buttons get dark overrides; data-bs-theme attribute set for other Bootstrap components

Test plan

• Click the moon/sun toggle — UI switches between light and dark themes
• Refresh the page — theme preference persists
• Clear localStorage theme key, set OS to dark mode, refresh — should default to dark
• Ace editor switches between xcode and tomorrow_night themes
• Bootstrap buttons (Run, Shareable Link, links) adapt in both themes
• Output pane text and spinner are visible in both themes
• On screens ≤ 450px, toggle is hidden (matches existing menu bar behavior)

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