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/components/themes/ThemeListItem.vue

@@ -11,6 +11,7 @@ const emit = defineEmits<{
   delete: [id: string]
   clone: [id: string]
   setDefault: [id: string]
+  edit: [theme: Theme]
 }>()
 
 function getAccentColor(): string {
@@ -48,6 +49,18 @@ function getAccentColor(): string {
         </svg>
       </button>
 
+      <button
+        v-if="!theme.is_system"
+        class="action-btn"
+        @click="emit('edit', theme)"
+        title="Edit theme"
+      >
+        <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+          <path d="M11 4H4a2 2 0 0 0-2 2v14a2 2 0 0 0 2 2h14a2 2 0 0 0 2-2v-7"/>
+          <path d="M18.5 2.5a2.121 2.121 0 0 1 3 3L12 15l-4 1 1-4 9.5-9.5z"/>
+        </svg>
+      </button>
+
       <button
         class="action-btn"
         @click="emit('clone', theme.id)"