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/_components/tasks/task-dialog.tsx

@@ -220,10 +220,11 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
 
             {/* Title */}
             <div>
-              <label className="text-sm font-medium mb-2 block">
+              <label htmlFor="task-title" className="text-sm font-medium mb-2 block">
                 Titel <span className="text-destructive">*</span>
               </label>
               <Input
+                id="task-title"
                 {...form.register('title')}
                 aria-invalid={!!form.formState.errors.title}
                 autoFocus
@@ -240,13 +241,14 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
 
             {/* Description */}
             <div>
-              <label className="text-sm font-medium mb-2 block">Omschrijving</label>
+              <label htmlFor="task-description" className="text-sm font-medium mb-2 block">Omschrijving</label>
               <Controller
                 control={form.control}
                 name="description"
                 render={({ field }) => (
                   <>
                     <TextareaAutosize
+                      id="task-description"
                       {...field}
                       value={field.value ?? ''}
                       aria-invalid={!!form.formState.errors.description}
@@ -275,13 +277,14 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
 
             {/* Implementation plan */}
             <div>
-              <label className="text-sm font-medium mb-2 block">Implementatieplan</label>
+              <label htmlFor="task-implementation-plan" className="text-sm font-medium mb-2 block">Implementatieplan</label>
               <Controller
                 control={form.control}
                 name="implementation_plan"
                 render={({ field }) => (
                   <>
                     <TextareaAutosize
+                      id="task-implementation-plan"
                       {...field}
                       value={field.value ?? ''}
                       aria-invalid={!!form.formState.errors.implementation_plan}