More accurate cost and filtering on the messaging diagnostics pages

Fix

Week of June 20, 2026

A few robustness fixes to the messaging diagnostics pages.

Real $0 totals — when every message in the filtered view genuinely cost nothing, the cost summary now reads $0.00 instead of a dash (which had meant “cost unknown”).
Stricter date filters — a malformed since/until value is now ignored rather than being guessed at, so a bad value can’t silently hide rows.
No error on odd status filters — the Pending Deliveries page now falls back to its default view when given an unrecognized status filter instead of showing an error page.

No admin action required.