feat(ui): add resizable session sidebar (#228)

## Summary
- add shared sidebar-width helpers and persist the preferred desktop
sidebar width in the UI store
- add a desktop-only draggable resize handle between the session list
and main content pane
- clamp rendered width against the current layout while preserving the
stored preferred width across passive layout changes and harden drag
lifecycle edge cases

## Test Plan
- [x] `cd frontend && npm test --
src/lib/components/layout/sidebar-width.test.ts
src/lib/stores/ui.test.ts
src/lib/components/layout/ThreeColumnLayout.test.ts`
- [x] `cd frontend && npm run build`
- [x] `git diff --check`
- [x] local roborev 3-agent matrix review (`codex`, `claude-code`,
`gemini`) on `upstream/main..HEAD` with no medium+ findings
- [ ] `cd frontend && npm run check`  
  Existing unrelated upstream failures remain in:
  - `frontend/src/lib/components/sidebar/SessionItem.svelte`
  - `frontend/src/lib/utils/tool-params.ts`
  - `frontend/src/lib/components/content/ToolBlock.test.ts`
  - `frontend/src/lib/utils/model.test.ts`
  - `frontend/src/lib/virtual/VirtualizerTest.svelte`
  - `frontend/src/lib/virtual/CacheTestWrapper.svelte`

Closes #218.

Author: tpn