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/useDarkMode.ts

@@ -0,0 +1,26 @@
+const isDark = ref(false)
+
+export function useDarkMode() {
+  const toggle = () => {
+    isDark.value = !isDark.value
+    applyTheme()
+  }
+
+  const applyTheme = () => {
+    const theme = isDark.value ? 'mytheme-dark' : 'mytheme'
+    document.documentElement.setAttribute('data-theme', theme)
+    localStorage.setItem('theme', theme)
+  }
+
+  const init = () => {
+    const saved = localStorage.getItem('theme')
+    if (saved === 'mytheme-dark') {
+      isDark.value = true
+    } else if (!saved && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+      isDark.value = true
+    }
+    applyTheme()
+  }
+
+  return { isDark, toggle, init }
+}