feat(site): clarify why secret name is disabled when editing (#26553)

Adds helper text under the disabled `Name` field in the Edit secret
dialog so users understand why it can't be changed:

> Unique identifier (can’t be changed).

Matches the existing pattern used elsewhere (e.g. AI settings). The
secret name is the resource identifier in the API path (`PATCH
/api/v2/users/{user}/secrets/{name}`) and `UpdateUserSecretRequest` has
no `name` field, so rename isn't supported server-side.

Created on behalf of @matifali.

---------

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

site/src/pages/UserSettingsPage/SecretsPage/SecretDialog.tsx

@@ -233,7 +233,11 @@ const SecretFields: FC<SecretFieldsProps> = ({
 	return (
 		<>
 			<FormField
-				field={getFieldHelpers("name")}
+				field={getFieldHelpers("name", {
+					helperText: disableName
+						? "Unique identifier (can’t be changed)."
+						: undefined,
+				})}
 				label={
 					showRequiredLabels ? (
 						<RequiredFieldLabel>Name</RequiredFieldLabel>

site/src/pages/UserSettingsPage/SecretsPage/SecretsPageView.stories.tsx

@@ -298,6 +298,9 @@ export const EditDialogOpened: Story = {
 		).toBeInTheDocument();
 		await expect(dialogView.getByLabelText("Name")).toHaveValue(secret.name);
 		await expect(dialogView.getByLabelText("Name")).toBeDisabled();
+		await expect(
+			dialogView.getByText("Unique identifier (can’t be changed)."),
+		).toBeInTheDocument();
 		await expect(dialogView.getByLabelText("Description")).toHaveValue(
 			secret.description,
 		);