fix(quick-260921-bi2): a11y - Layout-Navigation und Einstellungsformulare

useButtonType: Hamburger-, Profil-, Kategorie-, Ein-/Ausklapp- und Logout-
Buttons (kein Formular in diesen Dateien) erhalten type="button".

noSvgWithoutTitle: Icons neben sichtbarem Text werden dekorativ
(aria-hidden="true"). Zwei bislang unbenannte interaktive Elemente erhalten
zusaetzlich ein aria-label, weil ihr Text im eingeklappten Sidebar-Zustand
verschwindet bzw. ganz fehlte: die Dashboard-/Marketplace-Links und der
"Einstellungen"-Button in der Seitenleiste (t('dashboard')/t('marketplace')/
t('settings'), alle bereits vorhandene Schluessel) sowie der mobile
Schliessen-Button (tCommon('close')).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TPPB4ApQxzSU1rwV2Ffj9J

apps/web/src/components/layout/header.tsx

@@ -99,11 +99,13 @@ export function Header() {
       {/* Left: Logo + Mobile hamburger */}
       <div className="flex items-center gap-3">
         <button
+          type="button"
           onClick={() => setMobileOpen(true)}
           className="inline-flex items-center justify-center rounded-md p-2 text-muted-foreground hover:bg-muted hover:text-foreground transition-colors md:hidden"
           aria-label={t('common.menu')}
         >
           <svg
+            aria-hidden="true"
             xmlns="http://www.w3.org/2000/svg"
             width="20"
             height="20"
@@ -145,6 +147,7 @@ export function Header() {
         {/* User avatar dropdown */}
         <div className="relative" ref={dropdownRef}>
           <button
+            type="button"
             onClick={() => setDropdownOpen(!dropdownOpen)}
             className="h-8 w-8 rounded-full overflow-hidden bg-primary flex items-center justify-center text-xs font-medium text-primary-foreground hover:opacity-90 transition-opacity"
             aria-label={tHeader('userMenu')}
@@ -192,6 +195,7 @@ export function Header() {
                   className="flex w-full items-center gap-2 rounded-md px-2 py-1.5 text-sm text-foreground hover:bg-muted transition-colors"
                 >
                   <svg
+                    aria-hidden="true"
                     xmlns="http://www.w3.org/2000/svg"
                     width="16"
                     height="16"
@@ -221,7 +225,7 @@ export function Header() {
                         : 'text-foreground hover:bg-muted'
                     }`}
                   >
-                    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+                    <svg aria-hidden="true" xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
                       <path d="M12 22s8-4 8-10V5l-8-3-8 3v7c0 6 8 10 8 10z" />
                     </svg>
                     {tHeader('admin.label')}
@@ -233,6 +237,7 @@ export function Header() {
               <div className="border-t border-border my-1" />
               <div className="px-2 py-1">
                 <button
+                  type="button"
                   onClick={() => {
                     setDropdownOpen(false);
                     logout();
@@ -240,6 +245,7 @@ export function Header() {
                   className="flex w-full items-center gap-2 rounded-md px-2 py-1.5 text-sm text-foreground hover:bg-muted transition-colors"
                 >
                   <svg
+                    aria-hidden="true"
                     xmlns="http://www.w3.org/2000/svg"
                     width="16"
                     height="16"