feat(app): library-first shell with permanent Sidebar

[graydawnc]

Why

Spool's old shell was a single-pane search box with results filling the whole window. As the product moves toward "your AI session library", the right primary surface is browsing — by project — with search as a dedicated overlay. That requires a permanent Sidebar listing every project, with one-click drill-in.

What this PR does

Adds a Sidebar component driven by project_groups_v (introduced earlier in this stack) and the IPC plumbing to expose project groups to the renderer. The right pane now swaps content based on what was clicked: home feed, project view, search results, or session detail.

The window's minWidth is bumped to 800px because the sidebar plus content needs more horizontal real estate than the old single-pane layout — anything narrower starts truncating project names aggressively. This is a deliberate trade-off: smaller windows are not a primary use case for a session library.

How it connects

This is the visual skeleton. It is intentionally light on content — the project page, search overlay, and library landing all land in subsequent PRs. Submitting them on top of this branch keeps each one reviewable in isolation.

The Sidebar talks only to project_groups_v. It does not know about session content; clicking a row dispatches an identity_key upward and lets the parent decide what to render in the content pane.

Test plan

• e2e covers sidebar visibility and project selection
• Manual: switch between projects via sidebar; resize window to verify minWidth clamp; confirm project counts match the database