Rate fetching is optional and fails soft: any network or parse error
leaves cached rates untouched and shows a message, never throwing into
render. A cleared or non-positive FX rate input now calls the new
removeFxRate action instead of storing 0, since toBase() silently
converts a stored 0 to a 0-value conversion while an absent row
correctly throws MissingRateError. Posts are archived, never deleted.
Import downloads a backup of the current data before replacing it and
requires confirmation showing before/after counts.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
