F3: Sidebar hierarchy (Client > Project > Thread tree)

[rororowyourboat]

Summary

Replace the current flat project list in the sidebar with a three-level navigation tree: Client > Project > Thread. This replaces the client-side-only project-group UI with a server-backed hierarchy.

Scope

• Refactor Sidebar.tsx to render Clients as top-level collapsible sections
• Each Client section contains its assigned Projects (existing SortableProjectItem adapted)
• Each Project contains its Threads (existing behavior preserved)
• Add "Unassigned" section at the bottom for projects with no clientId (hidden when empty)
• Client sections show: name, project count badge, collapse/expand toggle, context menu (rename, archive, delete)
• Collapse/expand state for Client sections stored in localStorage (UI-only, via uiStateStore)
• Read clients[] from Zustand store (synced via snapshot)
• Sort clients by sortOrder, projects by user-selected sort mode, threads unchanged

Design details

• Empty Client section: show subtle empty state + "New Project" affordance
• Zero Clients: show prompt to create first Client + any unassigned Projects
• Search/filter active: hierarchy flattens to matching results
• Keyboard navigation: support traversal across all three levels (Client → Project → Thread)
• Preserve existing status indicators (terminal running, PR open, unread) on Thread items

Key files to modify

• apps/web/src/components/Sidebar.tsx — major refactor, nested rendering
• apps/web/src/components/Sidebar.logic.ts — update sort/group logic to work with Client hierarchy
• apps/web/src/uiStateStore.ts — add Client expand/collapse state
• apps/web/src/store.ts — ensure clients[] from snapshot is accessible
• apps/web/src/types.ts — add Client frontend type if needed

Dependencies

• F1 (Client entity), F2 (Client-Project binding)

Acceptance criteria

• Sidebar renders Client > Project > Thread hierarchy
• Clients are collapsible, state persisted in localStorage
• "Unassigned" section shows projects without a Client
• "Unassigned" section hidden when empty
• Context menu on Client headers (rename, archive, delete)
• "New Client" button in sidebar
• Search still works across the hierarchy
• Thread status indicators preserved

[rororowyourboat]

Implemented in feat/sidebar-project-groups branch.