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/root.tsx

@@ -68,7 +68,7 @@ export const Layout = ({ children }: { children: React.ReactNode }) => (
       <Meta />
       <Links />
     </head>
-    <body className="bg-[#11110f]">
+    <body className="bg-[#0e0e0d]">
       {children}
       <ScrollRestoration />
       <Scripts />
@@ -81,8 +81,8 @@ const App = () => (
     <AuthenticatedFlueProvider>
       <ThemeProvider
         attribute="class"
-        defaultTheme="light"
-        forcedTheme="light"
+        defaultTheme="dark"
+        forcedTheme="dark"
         disableTransitionOnChange
         storageKey="vite-ui-theme"
       >