Credentials tree phone indent depth visibility

Found while visually checking Task 9404 (lazy credentials tree) at 448px. Suggestions: 1. At 448px a folder at depth 4+ (e.g. Operator › Shared › viz-9404 › aws › prod) draws its disclosure at the same x as its parent (`aws` and `prod` align), so a reader cannot tell child from sibling; keep a visible indent step per level (or a rail/guide) on the phone band in `ui/feed/FeedList` for tree rows. 2. Check the same on other tree rosters (Automation > Schedules, Playbooks), which share the feed engine. Out of scope: desktop indentation (fine), the tree's data loading (Task 9404). Done means: - At 448×921, each tree depth up to at least 6 is visually distinguishable from its parent (screenshot in the PR). - Desktop rendering unchanged.

Phone tree indent now steps 6px per level past depth 3, so depths 4–6 no longer sit on the parent's column; desktop unchanged. Quick gate and changed-area e2e green. 448px visual check done: every level steps. PR #6776 queued in the lane. Memory: used mem-23434; proposed none.