feat(v0.5): home session chooser — no auto-created/restored tabs

Opening the app no longer auto-creates a blank tab or auto-restores/discovers
sessions as tabs. Instead it lands on a launcher (session chooser): a grid of the
host's running sessions as live preview thumbnails — pick one to open (replays its
full scrollback), or '+ New session'. Closing the last tab returns to the chooser.

- public/launcher.ts: mountLauncher() renders the chooser into the terminal area
  (live xterm thumbnails via /live-sessions + /preview, 5s refresh), New session +
  Manage link
- tabs.ts: drop restore()/syncLiveSessions() auto-tab behavior; constructor lands
  on the launcher; rebuild() toggles launcher when tab count is 0; closeTab no
  longer spawns a blank tab
- style.css: .launcher chrome (reuses the .mg-* card/grid/thumbnail styles)

Verified: fresh load → 0 tabs, chooser with thumbnails; Open → tab + chooser
hides; close last tab → chooser returns. 228 tests green, tsc + build clean.

public/style.css

@@ -660,6 +660,50 @@ body {
   color: #2a0808;
 }
 
+/* ── Launcher (home session chooser, shown when no tab is open) ──── */
+.launcher {
+  position: absolute;
+  inset: 0;
+  overflow-y: auto;
+  padding: 22px 18px 40px;
+  box-sizing: border-box;
+  background: var(--bg);
+}
+.launcher-head {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 10px 16px;
+  margin-bottom: 18px;
+}
+.launcher-title {
+  font-size: 20px;
+  font-weight: 700;
+  color: #fff;
+}
+.launcher-sub {
+  flex: 1 1 auto;
+  color: var(--text-dim);
+  font-size: 13px;
+}
+.launcher-actions {
+  display: flex;
+  gap: 8px;
+}
+.launcher-new {
+  background: var(--accent);
+  color: #fff;
+  border: none;
+  border-radius: 8px;
+  padding: 9px 16px;
+  font: inherit;
+  font-weight: 600;
+  cursor: pointer;
+}
+.launcher-new:hover {
+  background: var(--accent-2);
+}
+
 /* ── Session Manager page (manage.html) ──────────────────────────── */
 #manage-root {
   max-width: 1200px;