NodeLink storage settings: UI follow-ups from Task

Found while rendering Settings › Infrastructure › NodeLink storage locally (Task 6749), desktop and 390px. 1. The location form's Save stays disabled with no hint of what is missing (Region, Subnet, Allowed CIDR, credential). Name the missing fields under the button, or mark them required. 2. Region is typed free-text with only an "e.g. us-east-1" hint, so a typo is caught only when AWS answers on Save. Offer the AWS region list as a picker, the same as cloud nodes do. 3. The location card has no "Show policy" view of the live document, unlike the org Storage tab. Let the operator expand the policy behind the new Policy fact (`GET …/nodelink-locations/{location}/policy` already returns it). 4. When Task 6701 adds the org Storage tab's Policy Current/Outdated fact, it should call `nodelinks.policy_is_current`, so the two surfaces cannot disagree. 5. The card shows "Unreadable" with AWS's reason, but not what to do next. When the reason is AccessDenied, name the missing IAM action (DescribeFileSystemPolicy) in the caption. Out of scope: the EFS policy document itself; IAM grants. Done means: each item above is shipped or explicitly declined with a reason; `./dc check full` green.

Shipped items 1 (Save names missing fields), 2 (Region picker) and 5 (AccessDenied names the IAM action). Items 3 and 4 were already on develop (6749/6701). PR #5689 merged and deployed. The local full gate was killed for low memory; targeted suites pass. Visual check OK at 1440px and 390px. Follow-up #6887.