Add resizable chat sidebar

[shivamhwp]

Closes #213

What Changed

• Added desktop resize support to the main left chat sidebar.
• Reused the existing shared Sidebar resize rail instead of introducing a new layout system.
• Persisted the sidebar width in local storage with chat_thread_sidebar_width.
• Added a minimum remaining width check for the main content area so resizing cannot collapse the chat view into an unusable layout.
• Kept mobile/sidebar sheet behavior unchanged.

Why

Issue threads requested a resizable sidebar because long project and thread names are getting truncated. The codebase already had a resize-capable sidebar primitive used by the inline diff panel, so the lowest-risk approach was to wire the main chat sidebar into that existing mechanism.

This keeps the change small, consistent with existing behavior, and avoids adding new dependencies or maintaining a second resize implementation. The width guard is there to preserve predictable layout behavior under narrower desktop viewports.

UI Changes

• Before: left sidebar width was fixed.
• After:

resizable-sidebar.mov

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Add resizable sidebar to chat layout

Makes the chat thread sidebar user-resizable via a drag rail. The sidebar persists its width to localStorage under the key chat_thread_sidebar_width, enforces a minimum sidebar width of 256px, and rejects resize attempts that would shrink the main content area below 640px.

^{Macroscope summarized bb0fe0d.}

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