fix(web): make the inline rail resizable via touch and stylus

[btli]

Part of #4790

Related issue

Implements TR-6 / TR-7 / TR-9 of the touch interaction spec (the touch-interaction spec (see #4790)) for the always-visible inline rail. No standalone issue to close.

Summary

The right-rail resize handle only listened for mouse events on a 1px target, so touch and stylus drags lost the pointer as soon as they left the handle (or crossed the HTML-preview iframe).

• Migrate useResizableInlinePanel from mouse to pointer events: onPointerDown + setPointerCapture, with move/up on the captured element.
• Abort cleanly on pointercancel and lostpointercapture (last applied size, no persist, listeners gone). First pointer wins until that drag ends. Unmount still drops the iframe overlay.
• Touch affordances live only on returned handleProps: touch-action: none plus 20px content-box padding each side (44px hit target on the existing w-1 handle). WorkspacePanel / AppShell still spread handleProps unchanged. Width gating is untouched.

ELI5

The right-rail divider still looks like a thin line. The invisible grab area around it is now finger-sized, and once you start dragging, that finger owns the resize until you lift it (or the browser cancels).

sequenceDiagram
  participant Finger
  participant Handle
  Finger->>Handle: pointerdown
  Handle->>Handle: setPointerCapture
  loop while captured
    Finger->>Handle: pointermove (even off-handle / over iframe)
    Handle->>Handle: update live width
  end
  alt pointerup
    Finger->>Handle: persist width
  else pointercancel / lostpointercapture
    Finger->>Handle: abort, keep last applied, no persist
  end

Loading

Test Plan

• npx pnpm install --frozen-lockfile at worktree root (Node 22)
• web/node_modules/.bin/vitest run src/hooks/useResizableInlinePanel.test.tsx — 22 passed
• web/node_modules/.bin/vitest run src/shell/WorkspacePanel.test.tsx — 41 passed
• web/node_modules/.bin/vitest run src/shell/AppShell.test.tsx — 102 passed
• web/node_modules/.bin/tsc -b — clean
• Mutation-check: revert the hook to mouse events; the new capture / pointercancel / capture-loss / first-pointer-wins / affordance tests fail; restore and they pass
• Manual: on a touch or stylus device, drag the right-rail divider, continue the drag off the handle and over the HTML preview, lift — width should follow and persist. A second finger during the drag should be ignored. Browser gesture cancel should stop the drag without a stuck overlay.

Demo

Inline seam touch-drag validated live — foldable, tablet. Adjacent transcript scroll never resizes; gutter clears the scrollbar thumb band.

Type of change

• Bug fix
• Feature
• UI / frontend change
• Refactor / chore
• Docs
• Test / CI
• Breaking change

Test coverage

• Unit tests added / updated
• Integration tests added / updated
• E2E tests added / updated
• Manual verification completed
• Existing tests cover this change
• Not applicable

Coverage notes

Hook unit tests cover pointer capture, pointercancel abort, lostpointercapture abort, first-pointer-wins, 44px hit-target style, iframe overlay mount/unmount, and existing keyboard / reserved-width / persistence behavior. End-to-end touch demo is deferred to the consolidated validation phase.

Changelog

The right rail can be resized with touch and stylus, not just a mouse.

[btli]

Tribunal attestation (automated multi-engine review): CLEAN

Both blocking engines (codex + claude_code) attested head df3cf67b2. 3 review rounds on the production change plus re-attestations for the snapshot-baseline refresh and a test-only viewport pin (fixes a latent cross-PR interaction with #4785 found by integration validation: the AppShell separator test's 375px default fixture now correctly renders mobile layout under the new capability semantics, so the test pins 1024px). All findings fixed, none dismissed. Full record in .pollux/review-ledger.md.

[github-actions]

Closed. If you want to pick this back up, comment /reopen. GitHub only lets maintainers press the Reopen button, so this command does it for you. It needs the source branch to still exist.