fix(v0.4): mirror size-clamp bug + session manager page

The mirror looked broken because a backgrounded tab on one device still pinned
the shared PTY to its (default 80x24) size — so the device actively viewing got
a cramped terminal.

Fix: only an ACTIVELY-VIEWING client votes on PTY size.
- attachWs no longer seeds a default size vote (a join may be a hidden mirror)
- new 'blur' client message + clearClientDims(): a tab going hidden withdraws its
  size vote (still mirrors output); show() re-casts it
- PTY size = min cols/rows across clients that have actually reported dims
- session/protocol tests cover hidden-mirror-doesn't-clamp + blur

Session manager (separate page, for the 'too many sessions' problem):
- GET stays; add DELETE /live-sessions/:id and DELETE /live-sessions[?detached=1]
- manager.killById(); LiveSessionInfo gains cols/rows
- public/manage.html + manage.ts: list/open/kill sessions, kill-all / kill-detached,
  auto-refresh; 🗂 toolbar button; bundled as a 2nd esbuild entry

Verified: two concurrent clients mirror output + shared input; manage page
lists/kills (3→2→0). 225 tests green, tsc clean.

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 --bundle --format=esm --outdir=public/build --sourcemap",
-    "dev:web": "esbuild public/main.ts --bundle --format=esm --outdir=public/build --sourcemap --watch",
+    "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",
     "typecheck": "tsc -p tsconfig.json --noEmit && tsc -p tsconfig.web.json",
     "test": "vitest run",
     "test:watch": "vitest",