[BUG-94] LeanFin: show the full balance total on short time windows

[jmunar]

Summary

• Fix accounts silently dropping out of the balance evolution total: non-bank accounts (manual, Indexa) only get a balance snapshot when their value is updated, so an account with no entries inside the selected window produced an empty series and contributed nothing to the aggregated "All accounts" total.
• The window is now seeded with the most recent snapshot before the cutoff, so fill_balance_gaps carries the last known balance forward across the whole period.
• Exclude archived accounts from the aggregated series, matching the account dropdown on the same page — without this, closed accounts would now carry their last balance forward into the total forever.
• Document both behaviours in docs/requirements.md.

Test plan

• make check (fmt + clippy + full test suite) passes.
• New integration tests in crates/myapps-leanfin/tests/leanfin/balance_evolution.rs:
  • manual_account_without_recent_entries_still_shows_balance — an account last updated 200 days ago still renders its balance over a 90d window.
  • aggregated_series_includes_manual_account_without_recent_entries — total is 6000, not 1000.
  • aggregated_series_excludes_archived_accounts — an archived account does not inflate the total.
• Manually: on the Balance page, pick 30d/90d with "All accounts" selected while holding a manual account not updated in that period — the total now includes it.

Note: an account whose first ever snapshot falls inside the window (e.g. a bank connected 20 days ago viewed over 90d) still steps up on that date. That is genuinely unknown history rather than a missing carry-forward, so it is left as is.

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