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.

public/style.css

@@ -662,9 +662,12 @@ body {
 
 /* ── Session Manager page (manage.html) ──────────────────────────── */
 #manage-root {
-  max-width: 760px;
+  max-width: 1200px;
   margin: 0 auto;
   padding: 24px 16px 60px;
+  height: 100%;
+  overflow-y: auto;
+  box-sizing: border-box;
 }
 .mg-header {
   margin-bottom: 18px;
@@ -708,32 +711,54 @@ body {
   border-color: rgba(255, 107, 107, 0.5);
   color: var(--red);
 }
-.mg-list {
+.mg-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(330px, 1fr));
+  gap: 14px;
+}
+.mg-card {
   display: flex;
   flex-direction: column;
-  gap: 8px;
-}
-.mg-row {
-  display: flex;
-  align-items: center;
-  gap: 12px;
-  padding: 12px 14px;
+  gap: 10px;
+  padding: 12px;
   background: var(--surface-2);
   border: 1px solid var(--border);
-  border-radius: 10px;
+  border-radius: 12px;
 }
-.mg-main {
-  flex: 1 1 auto;
-  min-width: 0;
-}
-.mg-top {
+.mg-card-head {
   display: flex;
   align-items: center;
   gap: 8px;
 }
 .mg-name {
+  flex: 1 1 auto;
+  min-width: 0;
   color: #fff;
   font-weight: 600;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+/* Live preview thumbnail (scaled read-only xterm) */
+.mg-thumb {
+  position: relative;
+  width: 100%;
+  min-height: 90px;
+  background: #0e0f13;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  overflow: hidden;
+  cursor: pointer;
+}
+.mg-thumb:hover {
+  border-color: var(--accent);
+}
+.mg-thumb-inner {
+  position: absolute;
+  top: 0;
+  left: 0;
+  transform-origin: top left;
+  padding: 4px;
 }
 .mg-watch {
   font-size: 11px;
@@ -759,18 +784,20 @@ body {
 }
 .mg-meta {
   color: var(--text-faint);
-  font-size: 12px;
-  margin-top: 3px;
+  font-size: 11px;
   overflow: hidden;
   text-overflow: ellipsis;
   white-space: nowrap;
   font-family: Menlo, Consolas, monospace;
 }
 .mg-actions {
-  flex: none;
   display: flex;
   gap: 6px;
 }
+.mg-actions .mg-open {
+  flex: 1 1 auto;
+  text-align: center;
+}
 .mg-open {
   background: var(--accent);
   color: #fff;