Fix allowance-window reset timestamp jitter

[thrr87]

What changed

• align near-identical reset timestamps once at the analytics entrypoint
• anchor the current window to the live account reset and historical windows to their latest observation
• keep persisted observations unchanged
• add a regression test for the fake second Usage Remaining line

Root cause

Synced Macs can receive reset timestamps that differ by seconds for the same weekly allowance window. Exact Date equality split those readings into separate chart and analytics windows.

Validation

• focused regression failed before the fix and passes after it
• full Swift suite: 565 tests passed
• existing known-reset coverage confirms separate weekly windows remain separate

Closes #67