feat(ui): add resizable session sidebar

[tpn]

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

• 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
• cd frontend && npm run build
• git diff --check
• 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.

[roborev-ci]

roborev: Combined Review (bc350c0)

Verdict: The code changes look solid, and no issues of medium
or higher severity were found.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (c4c2a0d)

Summary Verdict: The PR implements a persistent, resizable sidebar for the desktop layout, but requires a fix to correctly handle active resizing when the sidebar closes.

Medium

• Location: frontend/src/lib/components/layout/ThreeColumnLayout.svelte
  • Problem: An
    active resize is only cancelled when the viewport leaves desktop mode or the component unmounts. If ui.sidebarOpen becomes false during a drag, the handle is removed from the DOM but isResizing, the global pointer listeners, and body.sidebar-resizing remain active until a
    later pointer event arrives, so a closed sidebar can still keep mutating ui.sidebarWidth.
  • Fix: Call stopResizing() when the sidebar closes and add a regression test that closes the sidebar mid-drag.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[tpn]

Rebasing and addressing comments now, expect a push soon when done.

[roborev-ci]

roborev: Combined Review (0345afc)

Verdict: The PR successfully implements a resizable sidebar with persistence, but contains a Medium severity breakpoint mismatch that needs to be addressed.

Medium

• Location: frontend/src/lib/components /layout/sidebar-width.ts:6, frontend/src/lib/components/layout/ThreeColumnLayout.svelte
• Problem: The new resize/persistence logic uses a 960px "desktop" breakpoint, but the layout CSS still switches to the mobile overlay only at
  767px. Between 768px and 959px, the app remains in the desktop two-column layout while the sidebar width style is removed and the stored preference is ignored, so the "persisted sidebar width" feature silently stops working across that viewport range.
• **
  Fix:** Make the JS breakpoint match the actual layout breakpoint, or update the layout CSS to switch to the non-resizable/mobile behavior at 960px as well so the rendering and resize logic stay in sync.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (cfb764c)

Verdict: All reviewers agree the code is clean and introduces no issues.

The changes successfully implement a persistent, resizable sidebar layout with local storage persistence, bounded width clamping, and comprehensive pointer event handling without introducing any security or structural vulnerabilities.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)