fix(web): deduplicate bar chart hover updates

[xingkaixin]

Summary

• suppress repeated bar hover callbacks when the column and band are unchanged
• move daily usage hover state into the token bar subtree so unrelated cost, date, and accessibility content stays stable
• memoize token chart derivations across hover updates

Performance

• two pointer events over the same bar now emit one hover update instead of two
• a hover update no longer rereads unrelated daily rows

Test plan

• pnpm lint
• pnpm format:check
• pnpm test
• pnpm build

Issue: CS-192