core: static sites serve a nested directory's index.html

`try_files {path} {path}/index.html /index.html` for SPA sites and the
dashboard: a URL like /docs/setup/ serves that directory's own index.html
before falling back to the root one, so a multi-page static site works at its
sub-paths while single-page apps still get client-side routing.

Found uncommitted in the checkout, author unknown, and already live: wildpc
runs from this tree, so every Caddyfile generated since used it.

core/src/wildpc_core/generators/caddyfile.py

@@ -261,12 +261,15 @@ def _host_remote_block(label: str, host: str, target: str) -> list[str]:
 
 
 def _static_serve_lines(serve_dir: str, spa: bool, indent: str) -> list[str]:
-    """The `root`/`file_server` body for a static site. SPA sites get a fallback to
-    the root `index.html` (client-side routing); content sites (Hugo) get plain
-    `file_server`, which resolves directory indexes and 404s missing paths."""
+    """The `root`/`file_server` body for a static site. SPA sites try a nested
+    directory index (`{path}/index.html`) before falling back to the root
+    `index.html` (client-side routing) — so a multi-page static site keeps working
+    at `/sub/dir/` URLs while single-page apps still resolve deep routes. Content
+    sites (Hugo) get plain `file_server`, which resolves directory indexes and
+    404s missing paths."""
     lines = [f"{indent}root * {serve_dir}"]
     if spa:
-        lines.append(indent + "try_files {path} /index.html")
+        lines.append(indent + "try_files {path} {path}/index.html /index.html")
     lines.append(f"{indent}file_server")
     return lines
 
@@ -456,7 +459,7 @@ def generate_caddyfile_from_registry(
     lines += [
         "    handle {",
         f"        root * {root}",
-        "        try_files {path} /index.html",
+        "        try_files {path} {path}/index.html /index.html",
         "        file_server",
         "    }",
         "}",

core/tests/test_caddyfile.py

@@ -145,7 +145,7 @@ class TestAcmeMode:
         cf = generate_caddyfile_from_registry(_acme({}))
         assert "@host_wildpc host wildpc.example.com" in cf
         assert "root * /data/repos/wildpc/app/dist" in cf
-        assert "try_files {path} /index.html" in cf
+        assert "try_files {path} {path}/index.html /index.html" in cf
         assert "file_server" in cf