a11y: keyboard-accessible panel toggles, region landmarks in inspector

- Add role="button" tabindex="0" aria-expanded to .link-host-title
  in both static and JS-rendered panels (host panels + UniFi switches)
- Sync aria-expanded in togglePanel(), restoreCollapseState(),
  collapseAll(), and expandAll()
- Add keydown handler (Enter/Space) so panel headers are keyboard-operable
- Add role="region" aria-label to inspector main chassis area
- Add role="complementary" aria-label to inspector port detail panel
- Replace last inline date-parse in renderLinks() with _toIso() helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/inspector.html

@@ -14,10 +14,10 @@
 </div>
 
 <div class="inspector-layout">
-  <div class="inspector-main" id="inspector-main">
+  <div class="inspector-main" id="inspector-main" role="region" aria-label="Switch chassis diagrams">
     <div class="link-loading">Loading inspector data</div>
   </div>
-  <div class="inspector-panel" id="inspector-panel">
+  <div class="inspector-panel" id="inspector-panel" role="complementary" aria-label="Port detail panel">
     <div class="inspector-panel-inner" id="inspector-panel-inner"></div>
   </div>
 </div>