/precision reports browser→relay and read-back→browser hops on a clock the page aligns with the relay #482

Piece 3 of 3 of #458.

Done:

• [ ] A small src/ui/clock-sync.js runs the same four-timestamp ping over /live and keeps the page's offset to the relay.
• [ ] The relay measures browser→relay from each frame's t, corrected by that page's offset. The page reports read-back→browser from the status read time it receives, through the existing precision report frame.
• [ ] Both hops appear with p50/p95 in hops of GET /precision and in the admin Precision panel.
• [ ] Tests: the clock-sync math in a unit test, plus a relay test with a fake page whose clock is skewed.
• [ ] The page's added JavaScript stays small, and bun scripts/design-lint.ts is clean if src/web or src/ui is touched. bun --cwd Software/so101 test passes.