feat: Add update_theme functionality with UI support

Backend:
- Add PUT /api/themes/:id endpoint for updating existing themes

Store:
- Add updateTheme method to theme store

MCP:
- Add update_theme tool to modify name, description, or save current variables

UI:
- Add edit button to ThemeListItem (custom themes only)
- Add edit modal with name and description fields
- Support editing from both desktop sidebar and mobile dropdown

frontend/src/stores/theme.ts

@@ -138,6 +138,29 @@ export const useThemeStore = defineStore('theme', () => {
     }
   }
 
+  async function updateTheme(id: string, data: { name?: string; description?: string; variables?: ThemeVariables; metadata?: ThemeMetadata }) {
+    saving.value = true
+    try {
+      const res = await fetch(`${API_URL}/api/themes/${id}`, {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(data)
+      })
+      const result = await res.json()
+      if (result.error) {
+        error.value = result.error
+        return null
+      }
+      await fetchThemes()
+      return result
+    } catch (e) {
+      error.value = 'Error updating theme'
+      throw e
+    } finally {
+      saving.value = false
+    }
+  }
+
   async function deleteTheme(id: string) {
     try {
       const res = await fetch(`${API_URL}/api/themes/${id}`, { method: 'DELETE' })
@@ -281,6 +304,7 @@ export const useThemeStore = defineStore('theme', () => {
     fetchThemes,
     fetchDesignTokens,
     saveTheme,
+    updateTheme,
     deleteTheme,
     setDefaultTheme,
     cloneTheme,