feat: P3 — WebKit container process pinned to dashboard

Dashward now actually renders content on the dashboard workspace. The
container is a separate gjs subprocess (so a WebKit crash can't take
down gnome-shell), and the extension pins its window to the dashboard
slot via the WindowGuard whitelist.

container/src/main.ts: tiny GJS bootstrap that creates a borderless
fullscreen Gtk.Window with a WebKit2 WebView and loads dashboard.html
from a runtime directory passed in via argv. `GLib.set_prgname` happens
before any GTK init so Wayland's xdg-shell app_id matches
`top.hangmanlab.dashward.container` -- that's the wm_class fingerprint
the extension matches against.

extension/src/container-supervisor.ts: spawn the container via
Gio.Subprocess; pump its stdout/stderr into journal under `[container
stdout|stderr]` tags so we can diagnose WebKit crashes without
attaching; watch display::window-created for the app_id match; on
arrival, whitelist with WindowGuard before moving to dashboard (so the
move's window-added doesn't bounce); make_fullscreen; clear the cached
ref on the window's `unmanaged` signal. Dispose SIGTERMs the
subprocess. P3 explicitly skips auto-restart / exponential backoff and
DBus signaling -- those land in P4.

container/runtime/runtime.ts + styles.css: a "Dashward" placeholder
card on the 12-column grid so the dashboard workspace is visually
distinct from a regular workspace; widget mounting / edit mode is P5+.

Verified on the ubuntu2504-test VM: extension enables cleanly, dashboard
shows the placeholder, switching to/from dashboard works, ding's window
is still ignored. MESA/EGL stderr lines are VM-only software-rendering
fallback noise (no virgl).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/runtime/styles.css

@@ -1,16 +1,28 @@
 :root {
   --bg: #f5f5f7;
   --fg: #1c1c1e;
+  --fg-muted: rgba(28, 28, 30, 0.55);
   --grid-gap: 16px;
 }
 
 [data-theme="dark"] {
   --bg: #0f0f12;
   --fg: #f0f0f5;
+  --fg-muted: rgba(240, 240, 245, 0.55);
 }
 
-html, body { margin: 0; padding: 0; height: 100%; background: var(--bg); color: var(--fg); }
-body { font-family: ui-sans-serif, system-ui, -apple-system, sans-serif; }
+html, body {
+  margin: 0;
+  padding: 0;
+  height: 100%;
+  background: var(--bg);
+  color: var(--fg);
+}
+
+body {
+  font-family: ui-sans-serif, system-ui, -apple-system, 'Inter', sans-serif;
+  -webkit-font-smoothing: antialiased;
+}
 
 #grid {
   display: grid;
@@ -20,3 +32,32 @@ body { font-family: ui-sans-serif, system-ui, -apple-system, sans-serif; }
   min-height: 100%;
   box-sizing: border-box;
 }
+
+.placeholder {
+  grid-column: 1 / -1;
+  display: grid;
+  place-items: center;
+  align-content: center;
+  gap: 12px;
+  min-height: 80vh;
+  text-align: center;
+}
+
+.placeholder h1 {
+  font-size: clamp(3rem, 8vw, 6rem);
+  font-weight: 200;
+  letter-spacing: -0.02em;
+  margin: 0;
+}
+
+.placeholder .hint {
+  font-size: 1.25rem;
+  margin: 0;
+  opacity: 0.7;
+}
+
+.placeholder .meta {
+  font-size: 0.9rem;
+  margin: 0;
+  color: var(--fg-muted);
+}