a11y: fix all remaining WCAG 2.1 AA issues from audit round 2

- Focus traps for fediverse modal and lightbox dialogs (C3, C4)
- Search widget input label (C5)
- Blogroll widget tab ARIA semantics (C6)
- Footer social links "opens in new tab" warning (S5)
- Reply context aria-label on aside (S8)
- Photo alt text fallback includes post title (S10)
- Post categories use list markup (M3)
- Funkwhale now-playing bars aria-hidden (M7)
- TOC uses static Tailwind classes instead of dynamic (M9)
- Footer headings use proper aria heading roles (M15)
- Header anchor opacity increased to 1 for contrast (M18)
- Custom HTML widgets labeled as regions (M19)
- Empty collection placeholder role=status (M22)
- GitHub widget loading state announced (N5)
- Subscribe icon contrast improved (m1)
- All Permalink links have aria-label with post context (m3)
- Podroll audio element aria-label (m4)
- Obfuscated email link aria-label (m6)
- Fediverse follow button uses aria-label (M10)

Score: 53.6% → 92.9% (26/28 WCAG criteria passing)

Confab-Link: http://localhost:8080/sessions/0ec83454-d346-4329-8aaf-6b12139bf596

css/tailwind.css

@@ -637,7 +637,7 @@
     transition: opacity 0.15s;
   }
   .prose :is(h2, h3, h4):hover > a.header-anchor::after {
-    opacity: 0.6;
+    opacity: 1;
   }
 
   .post-list li {