fix(modal): sub-anchor click no longer closes the umbrella modal

Sub-anchor links render as <a href="#" data-sub-anchor>, which were also
matched by the AsciiDoc cross-reference handler (a[href^="#"]). Clicking a
sub-anchor therefore fired both its own handler AND a bogus
navigate('/anchor/') with an empty id. Harmless before, but since the
modal now closes when a non-anchor route resolves (#565), handleRoute
normalizes '/anchor/' → '/anchor' (not an anchor route) and closed the
umbrella modal — breaking the "navigate to sub-anchor / show back button"
E2E tests.

Exclude data-sub-anchor links from the cross-reference handler and ignore
empty ids, so sub-anchor clicks only load the sub-anchor and the modal +
back button stay put.

Verified: full E2E suite (35) green locally, incl. the two umbrella tests
that regressed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: raifdmueller

website/src/components/anchor-modal.js

@@ -289,12 +289,16 @@ export async function loadAnchorContent(anchorId) {
       details.setAttribute('open', '')
     })
 
-    // Convert internal AsciiDoc cross-reference links to router navigation
-    contentEl.querySelectorAll('a[href^="#"]').forEach((link) => {
+    // Convert internal AsciiDoc cross-reference links to router navigation.
+    // Exclude sub-anchor links (handled above) — they share the href="#" shape
+    // and would otherwise also fire navigate('/anchor/') with an empty id,
+    // which the router treats as a non-anchor route and closes the modal.
+    contentEl.querySelectorAll('a[href^="#"]:not([data-sub-anchor])').forEach((link) => {
       const href = link.getAttribute('href')
-      // Only process simple hash links (cross-references), not hash routes
+      // Only process real cross-references: a non-empty id, not a hash route.
       if (href && href.startsWith('#') && !href.startsWith('#/')) {
         const anchorId = href.substring(1) // Remove the '#'
+        if (!anchorId) return
         link.addEventListener('click', (e) => {
           e.preventDefault()
           // Navigate to the linked anchor