v0.15.0: Fix 6 reported bugs

[dannymcc]

Summary

Resolves 6 open bug reports covering calculation errors, 500 errors, broken navigation, stale data, and missing translations.

Bug Fixes

• US MPG calculation (US MPG not calculated properly  #82): get_consumption() and get_average_consumption() now accept a consumption_unit parameter — for mpg/mpg_us users, returns distance/volume instead of (volume/distance)*100
• Recurring expense 500 error (Add Recurring Expense returns 500 error - v0.14.0 #83): Made RecurringExpense.amount column nullable to match the form UI ("Leave blank if amount varies"). Includes database migration.
• French language admin panel navigation (French Language - Admin Panel Navigation not working #85): Fixed showSection() JS function that relied on implicit window.event (unsupported in Firefox). Now passes event explicitly with a selector-based fallback.
• Cheapest Fuel not updating on delete (Deleting Fuel Logs doesn't update "Cheapest Fuel" #88, Fuel data issues #93): Fuel log deletion now cleans up matching FuelPriceHistory entries so the dashboard widget reflects current data.
• Quick Fuel Entry missing stations (Fuel data issues #93): Removed .limit(10) from station query to show all stations, matching the full form's behaviour.
• Untranslated dropdown values (Translation not covering full site #89): Wrapped 100+ display labels across 13 constant lists with lazy_gettext() and regenerated translation catalogs for all 14 languages.

Other Changes

• Bumped version to 0.15.0
• Removed stale messages.pot from repository root

Test plan

• All 512 existing tests pass
• Test recurring expense creation with blank amount — should succeed without 500
• Test fuel log deletion — cheapest fuel widget should update
• Test consumption display with MPG units — should show correct values
• Test settings navigation in Firefox — all tabs should work
• Verify dropdown labels appear translated for non-English users

Closes #82, #83, #85, #88, #89, #93