[feat](web): serve the SPA and web actions on the single listener

Add the browser edge (contract rev 8.2/9): a Preact/Vite SPA under web/,
an embedded placeholder bundle, the /web/* auth and action handlers, and
the explicit browser-action allowlist. The SPA and the API share the one
plaintext :8080 listener; there is no second port and no peer edge, so
main.go also drops the mTLS listener and the identity mounts in the debug
compose fall away. /_manifest announces the web block only while the SPA is
mounted (honesty rule), always with the single service port.

The DSNs move to the shared database's scram-sha-256 credentials, matching
rev 9's removal of the PKI layer.

Committed web/bun.lock because web/package.json and vite.config.ts are new
in this commit. src/web/dist/index.html is a hand-written placeholder, not
a generated bundle: it keeps bare `go build`/`go vet` working from a clean
checkout, while /web/dist (the real build output) stays gitignored.

web/src/components/Banner.tsx

@@ -0,0 +1,15 @@
+import type { ComponentChildren } from 'preact';
+
+interface BannerProps {
+  kind: 'error' | 'warn' | 'info';
+  children: ComponentChildren;
+  role?: 'alert' | 'status';
+}
+
+export function Banner({ kind, children, role = 'alert' }: BannerProps) {
+  return (
+    <div className={`banner banner-${kind}`} role={role}>
+      {children}
+    </div>
+  );
+}