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/layouts/default.vue

@@ -123,9 +123,9 @@
                 </div>
             </aside>
 
-            <div class="h-full flex-1 flex flex-col min-h-0">
+            <div class="h-full flex-1 flex flex-col min-h-0 min-w-0">
                 <AppTopNav :user="auth.user" />
-                <main class="flex flex-1 flex-col overflow-y-auto bg-white px-4 pb-24 sm:px-8 lg:px-16">
+                <main class="flex flex-1 flex-col overflow-y-auto overflow-x-hidden bg-white px-4 pb-24 sm:px-8 lg:px-16">
                     <div aria-hidden="true" class="pointer-events-none sticky top-0 z-30 h-8 flex-shrink-0 bg-white sm:h-12" />
                     <slot/>
                 </main>