Mark local Cursor dollars in overview as estimates, not the bill

[ozymandiashh]

Summary

Addresses the or be visibly marked half of #1545.

Local Cursor rows are priced from IDE composer bubbles, which never see the context Cursor re-sends on every request — on the account that reported the issue the local figure was about 2% of the dashboard charge — yet overview printed it like any exact total. The import from #1558 answers this for the time an export covers; outside coverage nothing said the number was not the bill.

renderOverview now walks the same assistant calls it already sums and, whenever the period still holds local cursor/cursor-agent dollars that no import covers, prints one dim line under the bottom line, in the style of the existing carried/unattributed notes:

  includes $3.14 of Cursor priced from local files, not Cursor's bill — codeburn import cursor <file.csv> replaces it with the dashboard's own totals

• Covered local calls are already dropped at serve time (feat(cursor): import Cursor's usage CSV to replace local estimates #1558), so any non-imported Cursor call that survives the walk is an uncovered estimate; imported events carry the cursor-import: dedup key and never trigger the line.
• Works for every overview period, custom --from/--to ranges, and --provider/--project filters, since the detection lives in the renderer.
• Exports CURSOR_IMPORT_KEY_PREFIX from cursor-import.ts instead of duplicating the literal.
• Docs (docs/providers/cursor.md) and the Unreleased CHANGELOG entry updated.

Deliberately out of scope (happy to follow up if wanted): the today/month TUI dashboard and the menubar chip render from other paths; this PR covers the overview report surface, which the issue's CLI repro used.

Testing

• 3 new cases in tests/overview.test.ts: the line appears for local Cursor and Cursor Agent rows with the summed amount, stays silent for imported (cursor-import:-keyed) rows, and silent for non-Cursor providers.
• npx vitest run tests — full suite passes (4771 passed, 5 skipped).