Verify the new BlockProductionHeader's layout at mobile/tablet — only desktop was ever screenshotted

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Context

PR #3887 adds a 3-tile BlockProductionHeader (inter-block time / throughput / author decentralization) to the /blocks index page, rendered as a horizontal row on desktop. The PR's before/after evidence has exactly one screenshot pair, both desktop-only (2600x1520) — no tablet or mobile capture exists. The throughput tile carries a long secondary value ("ext/block … events/block"), which is exactly the kind of content that risks cramping, uneven wrapping, or auto-stacking oddly at 375-414px widths. This can't be confirmed broken from the evidence available — it's an unverified gap on an above-the-fold header component on a heavily-trafficked index page, not a confirmed bug.

Requirement

Confirm whether BlockProductionHeader renders correctly at mobile and tablet widths; fix it if it doesn't.

Deliverable

• File to check/fix: apps/ui/src/routes/blocks.index.tsx, BlockProductionHeader.
• What "done" looks like concretely: capture the component at mobile (375px) and tablet (768px) in both themes. If the 3-tile row cramps, wraps unevenly, or clips the throughput tile's secondary value, apply the same tile-reflow treatment used elsewhere in the app for similar 3-up stat rows; if it already renders cleanly, this issue closes with just the verification screenshots as evidence and no code change required.

Acceptance criteria

• Before/after (or "verified, no change needed" + screenshots) screenshot table exists at mobile (375px) and tablet (768px), both themes
• If a fix was needed, blocks.index.tsx appears in the diff and the throughput tile's secondary value is fully legible at all tested widths
• Desktop rendering (already correct) is unaffected

Non-goals

• Don't change the header's data source or add new tiles — this is purely a verify-and-fix-if-needed pass on existing content.

Size

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

Part of #2542.

[linear-code]

JSO-2880