feat(ui) : add create task button on my-tasks and responsive kanban columns

- Add "Créer une tâche" button on my-tasks page with mandatory project selector
- TaskModal now accepts optional projects prop for project selection in create mode
- Replace fixed-width kanban columns (w-72 shrink-0) with flexible layout (min-w-36 flex-1)
- Add min-w-0 and overflow-x-hidden on default layout to properly contain content
- Kanban now adapts to screen size from 1024px to 1920px+

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

frontend/pages/projects/[id]/index.vue

@@ -1,5 +1,5 @@
 <template>
-    <div>
+    <div class="min-w-0">
         <div class="sticky top-8 z-20 bg-white pb-4 sm:top-12">
             <div class="flex items-center justify-between gap-3">
                 <h1 class="text-xl font-bold text-primary-500 sm:text-2xl">{{ project?.name ?? '' }}</h1>
@@ -62,11 +62,11 @@
         </div>
 
         <!-- Kanban -->
-        <div class="mt-6 flex gap-4 overflow-x-auto pb-4">
+        <div class="mt-6 flex gap-3 overflow-x-auto pb-4">
             <div
                 v-for="status in statuses"
                 :key="status.id"
-                class="flex w-72 shrink-0 flex-col rounded-lg transition-colors"
+                class="flex min-w-36 flex-1 flex-col rounded-lg transition-colors"
                 :class="dragOverStatusId === status.id ? 'bg-neutral-200' : 'bg-neutral-50'"
                 @dragover.prevent
                 @dragenter.prevent="onDragEnter(status.id)"