feat(web): add collapsible archived section in session list

[junmo-kim]

Summary

When working across multiple machines, archived project groups accumulate
in the sidebar and push active sessions far apart, requiring excessive
scrolling.

• Within each machine group, archived project groups are now separated
  into a collapsible "Archived (N)" section (default: collapsed)
• The section only appears when both active and archived groups coexist
  in the same machine — if all groups are archived, no separator is shown
• Selecting an archived session auto-expands its section

Test plan

• Open session list with multiple machines, each having active and archived sessions
• Verify archived groups appear under "Archived (N)" toggle, collapsed by default
• Click toggle to expand/collapse archived section
• Select an archived session — verify section auto-expands
• Verify no section separator when all groups in a machine are archived
• Keyboard navigation: Tab to archived toggle, Enter/Space to toggle

[github-actions]

Findings

• [Major] Nested button in the project-group row introduces invalid interactive markup and an accessibility regression. ProjectGroupItem now renders the entire row as a <button> while still placing CopyPathButton inside it, and CopyPathButton itself renders a <button>. Browsers and assistive tech handle nested buttons inconsistently, so the copy affordance can become unfocusable or trigger the wrong control. Evidence web/src/components/SessionList.tsx:486, web/src/components/SessionList.tsx:497
  Suggested fix:

<div className="group/project sticky top-0 z-10 flex items-center gap-2 px-1 py-1.5 rounded-lg transition-colors hover:bg-[var(--app-subtle-bg)]">
    <button
        type="button"
        className="flex min-w-0 flex-1 items-center gap-2 text-left"
        onClick={onToggle}
        title={group.directory}
        aria-expanded={!isCollapsed}
    >
        <ChevronIcon className="h-3.5 w-3.5 text-[var(--app-hint)] shrink-0" collapsed={isCollapsed} />
        <span className="font-medium text-sm truncate flex-1">{group.displayName}</span>
        <span className="text-[11px] tabular-nums text-[var(--app-hint)] shrink-0">({group.sessions.length})</span>
    </button>
    <CopyPathButton path={group.directory} className="opacity-0 group-hover/project:opacity-100 transition-opacity duration-150" />
</div>

Summary

• Review mode: initial
• 1 finding: the new project-group header contains a nested <button>, which is invalid HTML and a keyboard/screen-reader regression.
• Residual risk: Not found in repo/docs: component coverage for the new archived-section toggle and auto-expand-on-selection behavior.

Testing

• Not run (automation)

HAPI Bot

[github-actions]

[MAJOR] ProjectGroupItem now uses a <button> for the whole row, but CopyPathButton inside this row still renders its own <button> (web/src/components/SessionList.tsx:497). Nested buttons are invalid HTML and often break keyboard focus / assistive-tech behavior. Keep the toggle button and copy button as siblings instead of parent and child.

Suggested fix:

<div className="group/project ...">
    <button
        type="button"
        className="flex min-w-0 flex-1 items-center gap-2 text-left"
        onClick={onToggle}
        title={group.directory}
        aria-expanded={!isCollapsed}
    >
        ...
    </button>
    <CopyPathButton path={group.directory} className="opacity-0 group-hover/project:opacity-100 transition-opacity duration-150" />
</div>

[github-actions]

Findings

• No additional diff-attributable issues identified in the current head.

Summary

• Review mode: follow-up after new commits
• No new findings on the latest diff.
• Residual risk: Not found in repo/docs or adjacent SessionList tests: coverage for the archived-section toggle, auto-expanding the archived section when the selected session is archived, and cleanup of stale archived-collapse state.

Testing

• Not run (automation): bun is not available in this runner, so bun run test web/src/components/SessionList.test.ts and bun run typecheck could not be executed.

HAPI Bot

[github-actions]

Findings

• No additional diff-attributable issues identified in the current head.

Summary

• Review mode: follow-up after new commits
• No new findings on the latest diff.
• Residual risk: Not found in adjacent SessionList tests: coverage for the archived-section toggle, auto-expanding the archived section when the selected session is archived, and cleanup of stale archived-collapse state.

Testing

• Not run (automation): bun is not available in this runner, so bun test web/src/components/SessionList.test.ts and bun run typecheck could not be executed.

HAPI Bot