feat: collapsible sidebar

[mishraprafful]

Closes #30

Summary

• Adds a collapse/expand toggle button to the sidebar header
• Collapsed state shrinks the sidebar to a slim 40px strip showing only the toggle button
• Animates with a smooth width transition (200ms ease-in-out)
• The chevron icon rotates 180° when collapsed to indicate direction

Test plan

• Click the collapse button — sidebar animates to slim strip
• Click again — sidebar expands back with full content
• Verify session list, archive accordion, and new session button are hidden when collapsed
• Verify the main content area fills the freed space

Further Improvements

#34

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[greptile-apps]

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[dakl]

Is there a keyboard shortcut for collapsing and expanding? We should add one. Also, when cmd is held, we should show a tooltip somewhere to indicate that "if you press these keys, expansion will happen". Like a little speech bubble, but very small.

[mishraprafful]

@dakl Added the keyboard shortcut and also updated the PR description with further improvements.