feat: P5 — first widget on screen (clock via SDK)

The dashboard finally has live content. `runtime.ts` now imports
`defineWidget` from `@dashward/widget-sdk` (added as a workspace
dependency of the container), defines a tabular-numerals clock widget
inline, and mounts it onto the 12-column grid via a generic
`mountWidget` + `createContext` pair that produces the SDK's
config/system/lifecycle/shell APIs.

container/runtime/runtime.ts
  - mountWidget(def, parent, instanceId, size): creates a grid card,
    applies grid-column/row span, builds the widget context, calls
    def.mount; wraps in try/catch so a mount failure visibly degrades
    the card (red border + error label) instead of breaking the page.
  - createContext: localStorage-backed config (persists per
    instanceId), theme reads dataset.theme and subscribes to the
    dashward:ThemeChanged CustomEvent (DBus signal forwarded from
    shell), shell.call delegates to window.__dashShell__. onMount
    fires via microtask; onUnmount handlers stored; visibility is a
    no-op until P7 (edit mode / workspace tracking).
  - clockDef: defineWidget<ClockConfig>; shadow DOM root, large
    tabular-numerals time string, re-renders on tick + config.onChange.

container/runtime/styles.css
  - Real card style: half-transparent backdrop-filter blur with a
    light/dark variant, rounded 18px corners, subtle border and shadow.
  - .widget-error fallback for failed mount.
  - grid-auto-rows: minmax(120px, auto) so widgets have a sensible
    minimum row height.

container/package.json
  - `@dashward/widget-sdk` is now a runtime dependency. esbuild
    bundles it into runtime.js.

Verified on ubuntu2504-test VM: clock card renders top-left at 4-wide
× 2-tall, ticks every second, follows system theme live via the P4
bridge (light → glass-white card, dark → glass-black card).

Layout persistence, edit mode, dynamic disk loading of widget bundles,
and iframe crash isolation per design §11 / §14 land in P6+.

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

pnpm-lock.yaml

@@ -16,6 +16,10 @@ importers:
         version: 5.9.3
 
   container:
+    dependencies:
+      '@dashward/widget-sdk':
+        specifier: workspace:*
+        version: link:../sdk
     devDependencies:
       '@girs/gio-2.0':
         specifier: ^2.88.0-4.0.0