a11y: fix WCAG 2.2 AA and Uutilsynet audit findings

Tag autocomplete combobox pattern (WCAG 2.1.1, 4.1.2, 4.1.3):
- Add role="combobox", aria-expanded, aria-haspopup to tag input
- Implement arrow key navigation (up/down) through suggestions
- Add Space key support alongside Enter for selecting tags
- Manage aria-activedescendant to track highlighted option
- Add Escape to close suggestions
- Add aria-live="polite" status region announcing suggestion count
- Add aria-selected state on options
- Tag suggestions now have stable IDs for activedescendant

Focus visibility (WCAG 2.4.7):
- Remove outline:none on tag suggestions, replace with visible
  2px solid outline on :focus-visible

Contrast (WCAG 1.4.3):
- Replace opacity:0.5 on disabled rows with muted text color
  and strikethrough on username (maintains 4.5:1 ratio)

Structure and semantics (WCAG 1.3.1):
- Fix heading hierarchy H1→H3 skip in import.html (now H2)
- Replace <nav> misuse for fave actions with div[role="group"]
- Add aria-label="Administrasjonsmeny" to admin dashboard nav
- Wrap admin users table in responsive scrollable region
- Remove redundant "Bilde for:" prefix from image alt text
- Make error page H1 descriptive: "Feil 404: Ikke funnet"
- Add .sr-only utility class for screen-reader-only content
- Add hreflang="en" to English-language external link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/pages/fave_detail.html

@@ -28,7 +28,7 @@
         {{with .Fave}}
             {{if .ImagePath}}
                 <img src="{{basePath}}/uploads/{{.ImagePath}}"
-                     alt="Bilde for: {{.Description}}">
+                     alt="{{.Description}}">
             {{end}}
 
             <header>
@@ -56,7 +56,7 @@
             <footer>
                 <small>Lagt til {{.CreatedAt.Format "02.01.2006"}}</small>
                 {{if $d.IsOwner}}
-                    <nav class="fave-actions">
+                    <div class="fave-actions" role="group" aria-label="Handlinger for favoritt">
                         <a href="{{basePath}}/faves/{{.ID}}/edit" role="button" class="outline">Rediger</a>
                         <button
                             hx-delete="{{basePath}}/faves/{{.ID}}"
@@ -65,7 +65,7 @@
                             class="outline secondary"
                             data-redirect="{{basePath}}/faves"
                         >Slett</button>
-                    </nav>
+                    </div>
                 {{end}}
             </footer>
         {{end}}