feat: resizable sidebar/session pane

[tpn]

Summary

Add a draggable resize handle so the left-hand session/sidebar pane can be resized in the browser.

Problem

The session list can feel too narrow, especially for long titles, nested session structures, and metadata-heavy rows. Right now there is no visible splitter/resize affordance, so the pane width appears fixed.

Proposed Solution

Add a visible resize handle between the sidebar and the main content area.

Expected behavior:

• Hovering the divider shows an appropriate resize cursor and visible affordance
• Dragging with the mouse resizes the sidebar width live
• Reasonable min/max widths prevent layout breakage
• The chosen width persists across reloads if practical
• Works in normal browser usage and in the desktop app/webview

Acceptance Criteria

• Users can discover the resize interaction without guessing
• Sidebar width can be adjusted with the mouse
• Transcript/content area reflows correctly while resizing
• The interaction works on large sessions without visual glitches

Notes

I'm creating this issue with the intent to follow up with a PR.