Fix blank /web UI and /api route under the bundled server

The Flutter web bundle was built with the default base href of "/" but is
served under "/web/", so index.html loaded while every asset reference
resolved to the site root and 404'd, leaving a blank page. Build the bundle
with --base-href=/web/ so asset URLs match the mount point.

Also correct the root guidance text (the API explorer is at /api/docs, not
/api) and add an /api -> /api/docs redirect for convenience.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nix/frontend.nix

@@ -18,4 +18,10 @@ in
 
     # Build the web bundle (the backend serves these assets).
     targetFlutterPlatform = "web";
+
+    # The backend serves these assets under `/web/` (see web_server::serve), so
+    # the bundle must be built with a matching base href. Without this the built
+    # index.html keeps the default `<base href="/">` and every asset reference
+    # resolves to the site root, 404s, and the page renders blank.
+    flutterBuildFlags = ["--base-href=/web/"];
   }