fix(a11y): static accessibility findings (v1-readiness #4 — code-side)

Statische audit op happy-path-code; 4 categorieën gefixt vóór de Lighthouse-
verificatie die de gebruiker handmatig draait:

1. <main>-landmark op /login en /register (waren <div>); auth-pages krijgen
   nu een correcte landmark zodat screen-readers ze kunnen overslaan/nav

2. solo-task-card.tsx: agent-status-pill had role="button" + aria-label maar
   GEEN tabIndex en GEEN onKeyDown — keyboard-onbereikbaar. Nu compleet:
   tabIndex={0} + Enter/Space-handler

3. Form-label-associaties via htmlFor + id-pairs:
   - story-dialog (5): code, title, description, acceptance + priority via labelledby
   - task-dialog (3): title, description, implementation_plan
   - todo-list PromotePbi/PromoteStory dialogs (6): title, product, pbi, priority

   Lighthouse a11y "form-field-multiple-labels" en "label" rules worden
   hierdoor groen.

Niet aangeraakt:
- pbi-dialog: htmlFor was al goed gewired
- auth-form: htmlFor was al goed gewired
- Color-contrast: gebruikt MD3-tokens; theoretisch correct (verifieer in
  Lighthouse run)
- Heading-hierarchy: nog niet gescand — kan in vervolgronde

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/(auth)/login/page.tsx

@@ -5,7 +5,7 @@ import { QrLoginButton } from './qr-login-button'
 
 export default function LoginPage() {
   return (
-    <div className="min-h-screen bg-background flex items-center justify-center p-4">
+    <main className="min-h-screen bg-background flex items-center justify-center p-4">
       <div className="w-full max-w-sm space-y-6">
 
         {/* Logo / titel */}
@@ -42,6 +42,6 @@ export default function LoginPage() {
         </div>
 
       </div>
-    </div>
+    </main>
   )
 }

app/(auth)/register/page.tsx

@@ -4,7 +4,7 @@ import { AuthForm } from '@/components/auth/auth-form'
 
 export default function RegisterPage() {
   return (
-    <div className="min-h-screen bg-background flex items-center justify-center p-4">
+    <main className="min-h-screen bg-background flex items-center justify-center p-4">
       <div className="w-full max-w-sm space-y-6">
 
         {/* Logo / titel */}
@@ -26,6 +26,6 @@ export default function RegisterPage() {
         </div>
 
       </div>
-    </div>
+    </main>
   )
 }