feat(web): Work OS surface with Work Units, Signals, dual-mode composer

Lane C implementation of the Zopu Web Work OS for dogfooding:

- Work Unit card/detail projections built from per-issue events only
  (artifact counts from distinct artifact.updated paths, PR from
  gitea events, activity timeline, agent summaries). Signal linkage
  is explicitly 0/unavailable until Lane A relations are integrated.
- Persistent composer with Project and Work Unit mode switching.
  Project mode sends to the global Zopu agent; Work Unit mode sends
  to the issue-scoped project-manager agent identity.
- Collapsed Work Unit cards show title, summary, signal/step/artifact
  counts, current activity, PR indicator, needs-input indicator.
- Expanded Work Unit detail shows objective, timeline, artifacts,
  PR with directly usable link, needs-input alert, start action.
- Project-level Signals panel in the sidebar (not attributed to any
  issue until a signal-to-issue relation exists).
- Dark theme with calm, Apple-like visual direction.
- 21 projection tests including 5 cross-issue isolation tests and
  2 signal isolation tests proving one issue cannot inherit
  another issue's artifacts, PR, summary, or project signals.
- Mobile-responsive: detail overlay on mobile, sidebar on desktop.

No backend or schema changes. Uses existing Convex contracts and
Flue agent transport throughout.

apps/web/src/routes/app/dashboard/page.tsx

@@ -1,5 +1,5 @@
-import { ProjectWorkspacePage } from "@/components/projects/project-workspace-page";
+import { WorkOsPage } from "@/components/work-os/work-os-page";
 
 export default function Dashboard() {
-  return <ProjectWorkspacePage />;
+  return <WorkOsPage />;
 }