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/layouts/base.html

@@ -51,7 +51,7 @@
         <p>
             <small>
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-                — fri programvare under <a href="https://www.gnu.org/licenses/agpl-3.0.html" target="_blank" rel="noopener">AGPL-3.0</a>
+                — fri programvare under <a href="https://www.gnu.org/licenses/agpl-3.0.html" target="_blank" rel="noopener" hreflang="en">AGPL-3.0</a>
             </small>
         </p>
     </footer>