feat(ui) : add site colors, dark mode toggle and card styling improvements

- Site color field with color picker in create/edit modals
- Dark mode theme (mytheme-dark) with toggle in navbar
- Stronger site color visibility on cards (gradient, top border, badges)
- Bigger action buttons (btn-sm) on machine cards
- White card backgrounds with proper dark mode support

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/composables/useSiteManagement.ts

@@ -10,6 +10,7 @@ import { canPreviewDocument } from '~/utils/documentPreview'
 
 type SiteForm = {
   name: string
+  color: string
   contactName: string
   contactPhone: string
   contactAddress: string
@@ -31,6 +32,7 @@ type SiteDocument = {
 type SiteWithDocuments = {
   id: string
   name?: string
+  color?: string
   contactName?: string
   contactPhone?: string
   contactAddress?: string
@@ -54,6 +56,7 @@ export function useSiteManagement() {
 
   const newSite = reactive<SiteForm>({
     name: '',
+    color: '',
     contactName: '',
     contactPhone: '',
     contactAddress: '',
@@ -63,6 +66,7 @@ export function useSiteManagement() {
 
   const editSiteForm = reactive<SiteForm>({
     name: '',
+    color: '',
     contactName: '',
     contactPhone: '',
     contactAddress: '',
@@ -81,6 +85,7 @@ export function useSiteManagement() {
 
   const resetNewSite = () => {
     newSite.name = ''
+    newSite.color = ''
     newSite.contactName = ''
     newSite.contactPhone = ''
     newSite.contactAddress = ''
@@ -101,6 +106,7 @@ export function useSiteManagement() {
   const handleCreateSite = async () => {
     const result = await createSite({
       name: newSite.name,
+      color: newSite.color,
       contactName: newSite.contactName,
       contactPhone: newSite.contactPhone,
       contactAddress: newSite.contactAddress,
@@ -116,6 +122,7 @@ export function useSiteManagement() {
   const editSite = (site: SiteWithDocuments) => {
     siteBeingEdited.value = site
     editSiteForm.name = site.name || ''
+    editSiteForm.color = site.color || ''
     editSiteForm.contactName = site.contactName || ''
     editSiteForm.contactPhone = site.contactPhone || ''
     editSiteForm.contactAddress = site.contactAddress || ''
@@ -148,6 +155,7 @@ export function useSiteManagement() {
 
     const baseUpdate = {
       name: editSiteForm.name,
+      color: editSiteForm.color,
       contactName: editSiteForm.contactName,
       contactPhone: editSiteForm.contactPhone,
       contactAddress: editSiteForm.contactAddress,