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/package.json

@@ -13,6 +13,7 @@
     "dependencies": {
         "@nuxt/icon": "^2.2.1",
         "@nuxtjs/i18n": "^10.2.1",
+        "@malio/layer-ui": "^1.4.3",
         "@pinia/nuxt": "^0.11.3",
         "nuxt": "^4.3.0",
         "nuxt-toast": "^1.4.0",