Yield leaderboard drops the YIELD and VS MEDIAN columns entirely on mobile — the page's entire premise disappears

[JSONbored]

Context

On the 390px mobile screenshot supplied with #3708, the per-UID Yield leaderboard table shows only UID / HOTKEY / ROLE / STAKE T / EMISSION T. The YIELD percentage column and the VS MEDIAN directional-indicator column — the two columns that are the entire point of a page whose own PR description calls out "yield %, and an above/below-median indicator" — are missing from the rendered table on mobile. A phone user can see yield percentages in the three KPI tiles above the table, but not in the ranked leaderboard itself, which defeats the purpose of a ranked list on that viewport.

Requirement

The per-UID Yield leaderboard on mobile must still show yield and vs-median information for each row — either by keeping those columns visible (at the cost of some horizontal scroll, consistent with how other tables in the app handle mobile width) or by re-flowing the row into a mobile-card layout that surfaces yield/vs-median prominently per row, the way other leaderboard tables in this app already do below md.

Deliverable

• File to change: the Yield tab's per-UID leaderboard table component (introduced in feat(ui): add per-UID emission-yield tab to the subnet profile page #3708).
• What "done" looks like concretely: on a 390px viewport, each row in the leaderboard visibly communicates its yield percentage and its above/below-median status — via visible table columns (with horizontal scroll if needed) or via a card-style mobile layout that includes these two values per entry, matching whichever pattern the fix for the ListShell overflow issue establishes for this app's other tables.

Acceptance criteria

• The Yield leaderboard component appears in the diff
• At 390px mobile width, every row's yield percentage and vs-median indicator is reachable (visible directly, or via scroll/expand) — not silently dropped
• Desktop/tablet rendering is unaffected
• Includes a before/after screenshot table at mobile (390px or 375px), both themes, per the repo's UI-PR requirement

Non-goals

• Don't change the underlying yield calculation (see the separate, already-filed data-correctness issue about the leaderboard's identical-0.0016%-for-all-validators bug — that's a distinct concern from this layout gap).
• Don't remove or restyle the three KPI tiles above the table.

Size

Size: S — keep this PR small (aim for ≤10 files / ≤1000 LOC).

Part of #2542.

[linear-code]

JSO-2875