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/static/css/style.css

@@ -1,5 +1,18 @@
 /* Favoritter — custom styles on top of Pico CSS */
 
+/* Visually hidden, accessible to screen readers */
+.sr-only {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border: 0;
+}
+
 /* Skip navigation link for accessibility */
 .skip-link {
     position: absolute;
@@ -143,9 +156,14 @@
 }
 
 .tag-suggestion:hover,
-.tag-suggestion:focus {
+.tag-suggestion:focus-visible {
+    background: var(--pico-primary-focus);
+    outline: 2px solid var(--pico-primary);
+    outline-offset: -2px;
+}
+
+.tag-suggestion[aria-selected="true"] {
     background: var(--pico-primary-focus);
-    outline: none;
 }
 
 /* Fave detail actions */
@@ -177,7 +195,11 @@
 }
 
 .disabled-row {
-    opacity: 0.5;
+    color: var(--pico-muted-color);
+}
+
+.disabled-row td:first-child {
+    text-decoration: line-through;
 }
 
 .inline-input {
@@ -188,6 +210,11 @@
     font-size: 0.875rem;
 }
 
+/* Responsive admin tables */
+.table-responsive {
+    overflow-x: auto;
+}
+
 /* Respect reduced motion preference */
 @media (prefers-reduced-motion: reduce) {
     *, *::before, *::after {