feat(app): hover pill on resize handles and a wider sidebar range - #57343

Why

Kit compared our resize boundaries with Claude's desktop app. Claude shows a small pill on a boundary after a short hover, and it brightens while you drag. Its panes also resize across a much wider range. Ours showed nothing on hover. At a 1280 px window with the side panel open, the chat/side divider couldn't move at all: split diffs reserve 800 px for the side panel, so the chat panel was already at its 450 px floor.

What Changes

• One shared handle (packages/ui ResizeHandle): a 4 × 40 px pill centred on the handle. It fades in 200 ms into a hover (150 ms ease-out) in icon-muted, and brightens to icon-base while dragging. When the drag ends or the pointer leaves, it fades out over 110 ms. The hit area stays 8 px with col-resize/row-resize, and the cursor holds while dragging. It only changes opacity and colour, so nothing shifts.
• Handles sit in the gap: the sidebar handle and the bottom-pane handle now centre on the 8 px gap, as the chat/side handle already did. The file-tree and split-pane handles stay on their border line.
• Sidebar width: 140–520 → 120–640 px. At 120, Home still fits beside the sidebar toggle. Tab titles fade out as before, and Search/New session truncate.
• Chat ↔ side panel: a drag can now take either side down to 320 px (chat was 450; side was 480, or 800 with split diffs). The default width stays the same until you drag: the old 450/480/800 fit still applies to a never-resized session.

Scope

Desktop web/Electron layout only. No new settings, no API change and no theme token changes (uses the existing --v2-icon-icon-muted / --v2-icon-icon-base).

Verification

• bun typecheck (app, ui), bun test src/session/session-panel-width.test.ts with new cases for dragged widths, bun run lint:changed origin/beta.
• tabs.spec.ts now covers the new sidebar min (120) and max (640). The labels still fit at 140.
• Recorded against a hermetic server with the web build (fictional sessions). The clip and stills are on the board review: before | after at 1×, a 0.25× clip of the pill, and the narrowest sidebar. Measured pill timing in the 0.25× capture: it appears ≈210 ms after the hover starts, fades in over ≈150 ms and fades out over ≈100 ms.