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.

public/terminal-session.ts

@@ -351,6 +351,14 @@ export class TerminalSession {
 
   hide(): void {
     this.el.style.display = 'none'
+    // v0.4: withdraw our size vote so a backgrounded mirror doesn't clamp the
+    // shared PTY to this device's size. Output still streams (background mirror).
+    if (this.ws !== null && this.ws.readyState === WebSocket.OPEN) {
+      this.ws.send(buildMessage({ type: 'blur' }))
+    }
+    // Force show() to re-cast our dims even if unchanged.
+    this.lastCols = -1
+    this.lastRows = -1
   }
 
   /** Tear down: closing the WS detaches — the server PTY keeps running. */