feat: resizable left sidebar with hairline drag handle

Drag the right edge to resize (12–28rem), width persists in
localStorage. Hit area is 6px straddling the border; visual is a
1px hairline that lights up on hover and while dragging. Body
cursor + user-select are locked during drag so the resize cursor
stays put even if the pointer leaves the handle. Collapsed
icon-only state on the home page keeps its fixed width and hides
the handle.

Inner scroll wrapper added so the absolute-positioned handle stays
pinned to the viewport edge instead of scrolling with content.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>