Fix authentication validation to properly reject invalid CalDAV servers

- Backend: Enhance CalDAV discovery to require at least one valid 207 response
- Backend: Fail authentication if no valid CalDAV endpoints are found
- Frontend: Add token verification on app startup to validate stored tokens
- Frontend: Clear invalid tokens when login fails or token verification fails
- Frontend: Prevent users with invalid tokens from accessing calendar page

This resolves the issue where invalid servers (like google.com) were incorrectly
accepted as valid CalDAV servers, and ensures proper authentication flow.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/login.rs

@@ -145,6 +145,10 @@ pub fn Login(props: &LoginProps) -> Html {
                     }
                     Err(err) => {
                         web_sys::console::log_1(&format!("❌ Login failed: {}", err).into());
+                        // Clear any existing invalid tokens
+                        let _ = LocalStorage::delete("auth_token");
+                        let _ = LocalStorage::delete("session_token");
+                        let _ = LocalStorage::delete("caldav_credentials");
                         error_message.set(Some(err));
                         is_loading.set(false);
                     }