feat(v0.6): remove standalone Manage page — manage sessions on the Sessions view

The Sessions chooser already shows every running session with a live thumbnail
and Open + New session, so fold delete into it and drop the separate page:

- add a Kill ✕ button to each session card (DELETE /live-sessions/:id + refresh),
  reusing makePreviewCard's extraActions and the existing .mg-kill style
- remove public/manage.html + public/manage.ts and the esbuild manage entry
- remove the 🗂 toolbar button (main.ts) and the 🗂 Manage header link (launcher.ts)

Backend DELETE/preview routes stay (used by the launcher kill, thumbnails, and
the project pages). Frontend + build-config only.

Verified: full suite 453 green, web typecheck clean, build:web emits only main
(no manage.* artifacts), and in-browser the Sessions cards have Open + Kill (2→1
on kill), no Manage entry, and GET /manage.html → 404.

public/main.ts

@@ -76,16 +76,8 @@ mountHistory(toolbar, {
 mountShortcuts(toolbar)
 mountShareSession(toolbar, () => app.activeSessionId())
 
-// Session manager (standalone page) — manage/kill the host's running sessions.
-const manageBtn = document.createElement('button')
-manageBtn.className = 'toolbtn'
-manageBtn.textContent = '🗂'
-manageBtn.title = 'Manage sessions (open / kill)'
-manageBtn.setAttribute('aria-label', 'Manage sessions')
-manageBtn.addEventListener('click', () => {
-  location.href = '/manage.html'
-})
-toolbar.appendChild(manageBtn)
+// Session management lives on the home Sessions chooser now (open / kill /
+// new), so the standalone /manage.html page was removed.
 
 mountQrConnect(toolbar)