feat: add hashfs for static asset cache busting and live clock

- Add assets package with dev/prod build tags
- Dev: serve from filesystem with Cache-Control: no-store
- Prod: use hashfs for cache-busting URLs
- Add LiveClock component to show SSE connection status
- Update templates to use StaticPath for asset URLs

assets/assets.go

@@ -0,0 +1,5 @@
+// Package assets provides static file serving with build-tag switching
+// between live filesystem (dev) and embedded hashfs (prod).
+package assets
+
+const DirectoryPath = "assets"