perf: bypass React render cycle during panel resize drag

[joshhbk]

Summary

• During resize drag, CSS variables are now written directly to the DOM instead of calling setState on every mousemove, eliminating hundreds of React re-renders per drag
• Disables CSS grid transitions while dragging via an is-resizing class
• Prevents text selection flash during drag via preventDefault on mousedown
• React state and localStorage sync once on mouseup

Testing

• Drag each resizer (sidebar, right panel, plan, terminal, debug, chat/diff split). Handle should track cursor with no visible lag
• Release drag, reload page: confirm widths persist
• Existing text selections should remain duringn resize

Before

before_compressed.mov

After

after_compressed.mov

[Dimillian]

@codex review

[chatgpt-codex-connector]

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