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

server/index.ts

@@ -393,6 +393,47 @@ Bun.serve({
         return Response.json({ success: true }, { headers: corsHeaders })
       }
 
+      // PUT /api/themes/:id - Actualizar un tema existente
+      if (req.method === 'PUT' && !action) {
+        const theme = db.query('SELECT * FROM themes WHERE id = ?').get(id) as any
+        if (!theme) {
+          return Response.json({ error: 'Theme not found' }, { status: 404, headers: corsHeaders })
+        }
+
+        const body = await req.json()
+
+        // Build update query dynamically based on provided fields
+        const updates: string[] = []
+        const values: any[] = []
+
+        if (body.name !== undefined) {
+          updates.push('name = ?')
+          values.push(body.name)
+        }
+        if (body.description !== undefined) {
+          updates.push('description = ?')
+          values.push(body.description)
+        }
+        if (body.variables !== undefined) {
+          updates.push('variables = ?')
+          values.push(JSON.stringify(body.variables))
+        }
+        if (body.metadata !== undefined) {
+          updates.push('metadata = ?')
+          values.push(JSON.stringify(body.metadata))
+        }
+
+        if (updates.length > 0) {
+          updates.push('updated_at = CURRENT_TIMESTAMP')
+          values.push(id)
+
+          const sql = `UPDATE themes SET ${updates.join(', ')} WHERE id = ?`
+          db.run(sql, values)
+        }
+
+        return Response.json({ success: true, id }, { headers: corsHeaders })
+      }
+
       // GET /api/themes/:id - Obtener un tema
       if (req.method === 'GET' && !action) {
         const row = db.query('SELECT * FROM themes WHERE id = ?').get(id) as any