[expense] Mileage, per-category soft limit, currency display

[xuyushun441-sys]

Remaining increments for expense:

• Mileage: distance + mileage_rate fields with a mileage_amount formula + a Mileage seed line.
• Per-category soft limit: warning-severity validation referencing category.per_txn_limit (currently cosmetic).
• Currency display: show currency next to amounts in the grid + notify bodies (EUR report currently renders $).

Follow-up from the 9-template optimization + 9.5.1 runtime-testing pass.