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>

i18n/default_de.json

@@ -226,6 +226,7 @@
   "Dark": "Dunkel",
   "Dark green": "Dunkelgrün",
   "Toggle theme": "Theme umschalten",
+  "Main menu": "Hauptmenü",
   "Toggle Sidebar": "Sidebar umschalten",
   "Toggle Detail Sidebar": "Detail-Seitenleiste umschalten",
   "2FA Settings": "2FA Einstellungen",

i18n/default_en.json

@@ -226,6 +226,7 @@
   "Dark": "Dark",
   "Dark green": "Dark green",
   "Toggle theme": "Toggle theme",
+  "Main menu": "Main menu",
   "Toggle Sidebar": "Toggle Sidebar",
   "Toggle Detail Sidebar": "Toggle Detail Sidebar",
   "2FA Settings": "2FA Settings",