feat: add MCP apps sidebar to task threads

[fercgomes]

Summary

• Adds a collapsible right-side sidebar inside the task conversation view that lists every MCP-app (UI-rendering) tool call in the thread.
• Clicking an entry scrolls the chat to that exact item via the existing VirtualizedList.scrollToIndex mechanism (same as conversation search).
• A toggle button lives in the task header next to ExternalAppsOpener.
• A single global subscription to mcpApps.onDiscoveryComplete maintains a Set of tool keys with registered UI, so the sidebar filters to true "apps" without firing N hasUiForTool queries.

Files

New

• mcp-apps/stores/mcpUiToolsStore.ts — runtime registry of MCP tool keys with UI
• mcp-apps/hooks/useMcpUiToolsSubscription.ts — single global subscription that populates the registry
• sessions/stores/mcpAppsSidebarStore.ts — persisted open/width via createSidebarStore
• sessions/components/McpAppsSidebar.tsx — the panel with resize handle and clickable rows
• task-detail/components/McpAppsToggleButton.tsx — header icon button

Modified

• App.tsx — mounts the subscription once
• ConversationView.tsx — single pass derives both mcpAppIndices (for keepMounted) and mcpEntries; root wrapped in a Flex to host the sidebar
• TaskDetail.tsx — toggle button next to ExternalAppsOpener in the header

Test plan

• pnpm --filter code typecheck passes
• pnpm dev — open a task, generate a couple of PostHog MCP apps (insights / dashboard)
• Click the SquaresFour icon in the task header → sidebar opens to 260px on the right
• Sidebar lists only UI-rendering MCP calls; non-UI MCP tools are absent
• Scroll the chat up; click an entry → conversation scrolls to that item centred, iframe state preserved
• Drag the resize handle; reload the app → width persisted
• Close the sidebar; reload → closed state persisted
• Empty thread → sidebar shows empty state