Resizable sidebar (drag handle)

[dougss]

Sidebar today has fixed width. Add drag handle on right border to allow user resize:

• Drag handle on right border (cursor: ew-resize)
• Min width: 200px (preserves workspace + session readability)
• Max width: 500px (preserves main content area)
• Persist width in LocalStorage (deck:sidebarWidth)
• Snap-to-default option (double-click handle)

Useful especially with future Planner panel (#1) where horizontal space becomes premium.

Independent from Planner — sidebar resize works standalone.

[dougss]

Closed via commit. Refactored into reusable ResizablePanel component.