feat: app shell — activity bar + primary sidebar panel switching

[andrewchumchal]

Summary

Introduce the VS Code-style app shell as the new root layout. This is the structural foundation for Phase 2 — all other workspace model features (tabs, command palette, status bar, bottom panel) are built on top of it.

Current layout

┌──────────────┬────────────────────────┐
│   Sidebar    │      Chat area         │
│ (always open)│                        │
└──────────────┴────────────────────────┘

Target layout

┌───┬──────────────┬────────────────────┐
│ A │   Primary    │    Chat area       │
│ c │   Sidebar    │                    │
│ t │   (panel     │                    │
│ . │   switches)  │                    │
│ B │              │                    │
│ a │              ├────────────────────┤
│ r │              │  Bottom panel      │
├───┴──────────────┴────────────────────┤
│              Status bar               │
└───────────────────────────────────────┘

Scope

AppShell component

• New root layout component replacing the current top-level layout
• Manages the three-region structure: activity bar | primary sidebar | main area
• Sidebar collapse/expand preserved

Activity bar

• Narrow left icon strip (VS Code style)
• Icons: Chats, Personas, MCP, Settings
• Active icon highlighted; clicking switches the primary sidebar panel
• Clicking the active icon again collapses the sidebar (toggle behaviour)
• Uses brand colours and existing icon set

Primary sidebar panel switching

• Each activity bar icon maps to a panel:
  • Chats → current conversation list (existing Sidebar content)
  • Personas → persona list (existing SettingsPanel personas section, extracted)
  • MCP → MCP server list (existing SettingsPanel MCP section, extracted)
  • Settings → general settings (existing SettingsPanel general section)
• Panel state persisted in uiStore

uiStore changes

• activePanel: 'chats' | 'personas' | 'mcp' | 'settings'
• sidebarOpen: boolean

Acceptance criteria

• Activity bar renders on the far left with four icons
• Clicking each icon switches the primary sidebar panel
• Clicking the active icon collapses/expands the sidebar
• Panel selection survives app restart (persisted in uiStore)
• macOS traffic light clearance maintained (existing pl-[80px] logic)
• Existing functionality (conversation list, settings, MCP, personas) is preserved — just reorganised

Prerequisites

• None (this is the foundation)

Unlocks

• Tabbed conversations (feat: app shell — activity bar + primary sidebar panel switching #14 — tabs live in the main area header)
• Bottom panel slot (feat: status bar (model, token count, cost estimate) #17)
• Status bar (feat: command palette (⌘K) #16)
• Command palette (feat: tabbed conversations #15)