feat : migrate filter/form UI to @malio/layer-ui + fix hours/calendar contract scoping

- Add @malio/layer-ui as Nuxt layer (extends in nuxt.config.ts)
- Migrate site/employee/contract filters to MalioSelectCheckbox / MalioInputText / MalioSelect on employees, calendar and hours screens
- Migrate absence drawer selects + submit button to Malio (MalioSelect + MalioButton)
- Migrate calendar "Ajouter une absence" / "Imprimer" actions to MalioButton
- Drop now-unused EmployeeNameFilterInput and SiteFilterSelector components
- Hours day view: resolve contract nature at selected date (WorkHourDayContext.contractNature) instead of employee.currentContractNature (today-based); fixes interim contracts showing as CDI when mission ended
- Calendar: hide employees whose contract periods do not intersect the displayed month
- Layout: scrollbar-gutter:stable on <main> to avoid horizontal shift when dropdowns open
- Update functional-rules.md, in-app documentation, CLAUDE.md to match

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

frontend/layouts/default.vue

@@ -174,7 +174,7 @@
 
             <div class="h-full flex-1 overflow-hidden flex flex-col min-w-0">
                 <AppTopNav :user="auth.user" @toggle-sidebar="sidebarOpen = !sidebarOpen" />
-                <main class="flex-1 overflow-y-auto px-4 py-6 lg:px-8 lg:py-12">
+                <main class="flex-1 overflow-y-auto [scrollbar-gutter:stable] px-4 py-6 lg:px-8 lg:py-12">
                     <slot/>
                 </main>
             </div>