fix: restore LLM profile test connectivity buttons (lost from git)

Frontend test functionality was implemented but never committed to git.
Re-adds:
- profiles.js: testCard(), testForm(), _showTestResult(), test btn in renderCard
- api.js: testProfile(id) and probeConnectivity(body) methods
- index.html: 测试连通性 button + result div in profile form
- app.css: .btn-test and .profile-test-result styles

Backend /probe and /{id}/test endpoints were already present in llm_profiles.py.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

webapp/static/index.html

@@ -219,9 +219,11 @@
             </div>
             <div class="form-actions">
               <button class="btn btn-primary" id="save-profile-btn">保存</button>
+              <button class="btn btn-test" id="test-profile-btn">测试连通性</button>
               <button class="btn" id="cancel-profile-btn">取消</button>
               <span class="form-error muted" id="profile-form-error"></span>
             </div>
+            <div class="profile-test-result" id="profile-form-test-result" hidden></div>
           </div>
         </div>