fix: route project roots (/<slug>/) to a landing page

The home index and every page breadcrumb link a project root
`<a href="/<slug>/">`, but the router declared only `/`, `/assets/theme.css`,
and the `/{slug}/{*page}` catch-all (which requires a concrete page segment),
so every project-root link 404'd. Add a `/{slug}/` route and a project_index
handler that renders a landing page listing the project's sections and pages
(unknown slug -> 404); the index and breadcrumb root links are now live.

RED-first via the debug skill: tests/e2e.rs
`project_root_resolves_to_landing_with_section_nav` (a GET to /aura/ asserting
200 + the section/page nav) failed 404 before the fix, passes after.
cargo test green (42); clippy clean.

tests/e2e.rs

@@ -192,6 +192,24 @@ docs="docs"
     );
 }
 
+/// A registered project's root URL (`/<slug>/`) must resolve to a landing page
+/// that lists that project's sections/pages — not 404. Both the home index
+/// (`index`, src/server/mod.rs) and every page breadcrumb (`page_html`,
+/// src/theme/mod.rs) emit `<a href="/<slug>/">` for the project root, so it has
+/// to be a live route. The router declares only `/`, `/assets/theme.css`, and a
+/// `/{slug}/{*page}` catch-all that requires a concrete page segment, leaving a
+/// bare project root such as `/aura/` unrouted (404) while a concrete page like
+/// `/aura/design/INDEX` resolves — so every index and breadcrumb root link is dead.
+#[tokio::test]
+async fn project_root_resolves_to_landing_with_section_nav() {
+    let (status, body) = get(router(state()), "/aura/").await;
+    assert_eq!(status, StatusCode::OK, "a project root must resolve, not 404: {body}");
+    assert!(
+        body.contains("<a href=\"/aura/design/INDEX\">"),
+        "project landing must list the project's sections/pages: {body}"
+    );
+}
+
 /// A Fetcher that counts `fetch_raw` calls, wrapping a MockFetcher. Lets the
 /// E2E layer prove the cache-before-fetch data-flow.
 struct CountingFetcher {