(sidebar): every projects-changed rebuilds the whole sidebar, ~0.6 s with 900 sessions #543

Found by the rendering audit that followed #541. Measured on main 3a40242 in an isolated instance with a synthetic cache of 30 projects × 30 sessions (39,510 sidebar nodes).

renderProjects (public/sidebar.js:609-1104) rebuilds the whole sidebar on every projects-changed, even when nothing changed:

• the path is onProjectsChanged (app.js:1733-1746, 900 ms debounce) → loadProjects (app.js:1182);
• each call builds a new sidebar, runs morphdom over it, then rebindSidebarEvents;
• buildSessionItem (sidebar.js:1644-1790) creates about 15 SVG buttons per row through innerHTML.

Cost: 454 ms cold, 601–728 ms in steady state with identical data.

Style and layout afterwards cost 0–3 ms: the whole cost is JS, about 0.7 ms per session.

Frequency (inferred from the code, not observed with a real CLI): every JSONL write of a live session; the main-side watcher throttles to 1.5 s. That is about once a second while any session writes and the Sessions tab is shown.

Not measured: JB's real cache.

Fix, in order of payoff:

1. Skip renderProjects when the projects payload equals the previous one.
2. Clone a row <template> instead of 15 SVG innerHTML writes.
3. Re-render only the project groups whose sessions changed.