Feature Request: Resizable right sidebar panel

[himudigonda]

Allow users to drag the divider between the timeline and the right sidebar (day summary / activity detail) to resize it, with the chosen width persisting across app restarts.

Motivation

The right sidebar currently has a fixed width of 358px. Depending on the content — a long "Detailed Summary" in an activity card, or the donut chart legend with many categories — users may want the panel wider to read comfortably, or narrower to give the timeline more room. There's no way to adjust this today.

This is a standard interaction pattern in macOS apps (Finder, Xcode, Mail all have resizable sidebars), so users already know how to discover and use it.

Proposed behavior

• Drag the divider between the timeline and the right panel to resize.
• Cursor feedback: the standard macOS ↔ resize cursor appears on hover over the divider.
• Min width ~350px: prevents the DaySummaryView content (donut chart, legend labels, focus section) from overlapping or clipping.
• Max width ~40% of the panel: keeps the timeline column usable regardless of window size.
• Persistence: the chosen width is remembered across app restarts (via UserDefaults / AppStorage).
• No visual change at default: the divider stays as a 1px line; the drag hit area is invisible and wider for comfort.

I have a working implementation ready: #216

Two commits:

1. ui(main): add persisted sidebar width storage via AppStorage
2. ui(timeline): add draggable resize handle for right sidebar panel