fix: add P0 accessibility fixes to sidebar icon bar and search form

- Add :focus-visible outline to icon bar buttons/links (desktop + mobile)
- Add aria-label to icon bar <nav> for screen reader landmark distinction
- Add role="search" to sidebar search form
- Add "Main menu" / "Hauptmenü" translation key

Workflow: A11Y-SIDEBAR-P0-001

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

web/css/layout/app-aside-icon-bar.css

@@ -53,6 +53,14 @@ aside.aside-icon-bar li button.active {
   background: var(--app-icon-bar-hover-bg);
 }
 
+aside.aside-icon-bar li a:focus-visible,
+aside.aside-icon-bar li button:focus-visible {
+  outline: 2px solid var(--app-primary-focus);
+  outline-offset: -2px;
+  border-left-color: transparent;
+  color: var(--app-contrast);
+}
+
 @media (min-width: 768px) {
   aside.aside-icon-bar {
     border-right: var(--app-icon-bar-border);
@@ -106,6 +114,12 @@ aside.aside-icon-bar li button.active {
     border-top: var(--app-icon-bar-border-width) solid var(--app-primary);
   }
 
+  aside.aside-icon-bar li a:focus-visible,
+  aside.aside-icon-bar li button:focus-visible {
+    border-left-color: transparent;
+    border-top-color: transparent;
+  }
+
   .app-main-content,
   footer.site-footer {
     padding-bottom: max(