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.

package.json

@@ -12,8 +12,8 @@
     "start": "tsx src/server.ts",
     "dev": "tsx watch src/server.ts",
     "build": "tsc -p tsconfig.json",
-    "build:web": "esbuild public/main.ts public/manage.ts --bundle --format=esm --outdir=public/build --sourcemap",
-    "dev:web": "esbuild public/main.ts public/manage.ts --bundle --format=esm --outdir=public/build --sourcemap --watch",
+    "build:web": "esbuild public/main.ts --bundle --format=esm --outdir=public/build --sourcemap",
+    "dev:web": "esbuild public/main.ts --bundle --format=esm --outdir=public/build --sourcemap --watch",
     "typecheck": "tsc -p tsconfig.json --noEmit && tsc -p tsconfig.web.json",
     "test": "vitest run",
     "test:watch": "vitest",