feat(v0.4): session manager grid with live preview thumbnails

The flat session list didn't tell you what each session was doing. Rebuild
/manage.html as a full-page grid where each card renders a LIVE preview
thumbnail — a read-only xterm showing the session's actual current screen,
scaled down like a screenshot — so you can recognize each at a glance.

- RingBuffer.tail(maxBytes): newest scrollback on chunk boundaries (no ANSI/UTF-8
  split); GET /live-sessions/:id/preview returns {cols,rows,data} (no attach, so
  it doesn't inflate watcher counts or keep sessions alive)
- manage.ts: card grid; per card a scaled read-only Terminal renders CLEAR+tail,
  auto-refresh every 4s, click thumbnail/Open to join, Kill / bulk kill
- bundles xterm into manage.js (+manage.css linked in manage.html)
- ring-buffer tail tests

Verified: 3 sessions rendered as recognizable thumbnails (top / git log / ls),
colored, with status/age; click opens. 228 tests green, tsc clean.

src/types.ts

@@ -113,6 +113,9 @@ export interface RingBuffer {
   append(chunk: string): void;
   /** full buffer for replay; prepend `\x1b[0m` soft-reset as a safety net (M2). */
   snapshot(): string;
+  /** last ~maxBytes of recent output, on whole-chunk boundaries (never splits an
+   *  ANSI/UTF-8 sequence), for a read-only session preview. No soft-reset prefix. */
+  tail(maxBytes: number): string;
 }
 
 // impl anchor [src/session/ring-buffer.ts]: