feat: git mount URL validation with branch detection

- Add POST /config-profiles/validate-git-url endpoint:
  - Parses URL using existing parse_git_url utility
  - Suggests corrected URL for browser URLs
  - Runs git ls-remote --heads to verify reachability
  - Lists available branches from remote
  - Supports SSH key for private repos
  - Returns structured response: valid, suggested_url, branches,
    default_branch, error, error_code

- Update frontend GitMountEditor:
  - Add Check button next to URL field with loading state
  - Show validation result: valid (green), suggestion (yellow),
    invalid (red)
  - Suggestion includes Use this button to apply corrected URL
  - Branch field becomes dropdown when URL is validated,
    populated with remote branches
  - Mappings section disabled until URL is validated
  - Shows hint: Validate the URL first

- Quality gates: pytest (218 passed, 6 pre-existing),
  tsc --noEmit (clean)

apps/web/src/api/config_profiles.ts

@@ -167,3 +167,23 @@ export const resolveDefaultProfile = async (
 	});
 	return response.data;
 };
+
+export interface ValidateGitUrlResponse {
+	valid: boolean;
+	suggested_url?: string;
+	branches?: string[];
+	default_branch?: string;
+	error?: string;
+	error_code?: string;
+}
+
+export const validateGitUrl = async (
+	url: string,
+	sshKeyId?: string,
+): Promise<ValidateGitUrlResponse> => {
+	const response = await apiClient.post<ValidateGitUrlResponse>(
+		"/config-profiles/validate-git-url",
+		{ url, ssh_key_id: sshKeyId },
+	);
+	return response.data;
+};