Add selector parameter for element screenshots

- New optional selector parameter to capture specific DOM elements
- Works with all existing parameters (format, quality, darkMode, etc.)
- Validates selector for length (max 200 chars) and dangerous content
- Mutual exclusivity with fullPage parameter
- Returns 400 error if selector not found
- Updates OpenAPI spec for both GET and POST endpoints
- Full test coverage including edge cases

src/routes/screenshot.ts

@@ -117,6 +117,9 @@ export const screenshotRouter = Router();
  *             mobile:
  *               summary: Mobile viewport
  *               value: { "url": "https://example.com", "width": 375, "height": 812, "deviceScale": 2 }
+ *             element:
+ *               summary: Element screenshot
+ *               value: { "url": "https://github.com", "selector": "#readme" }
  *     responses:
  *       200:
  *         description: Screenshot image binary
@@ -263,6 +266,13 @@ export const screenshotRouter = Router();
  *           maxLength: 5000
  *         description: Custom JavaScript code to execute on the page before capture (max 5000 chars, 5-second timeout)
  *         example: "document.querySelector('.modal').remove();"
+ *       - name: selector
+ *         in: query
+ *         schema:
+ *           type: string
+ *           maxLength: 200
+ *         description: CSS selector for element to capture instead of full page/viewport (max 200 chars)
+ *         example: "#main-content"
  *       - name: darkMode
  *         in: query
  *         schema: