Add comprehensive accessibility improvements

HTML Accessibility:
- Add ARIA roles to tab navigation (role="tablist", role="tab", role="tabpanel")
- Add aria-selected to tab buttons with JS toggle
- Add aria-controls and aria-labelledby for tab/panel relationships
- Add aria-label to emoji icon buttons (settings, reply, edit, delete)
- Add aria-pressed to view toggle buttons
- Add labels for form inputs (comment textarea, dependency inputs, file input)
- Add .sr-only utility class for screen-reader-only content

CSS Accessibility:
- Add .sr-only class (visually hidden, accessible to screen readers)

JavaScript:
- Update showTab() to toggle aria-selected on tab buttons

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/dashboard.css

@@ -201,6 +201,19 @@ select:focus {
     outline-offset: 2px;
 }
 
+/* Screen reader only - visually hidden but accessible */
+.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;
+}
+
 /* Reduced motion for accessibility */
 @media (prefers-reduced-motion: reduce) {
     *,