Fix cheapest fuel not showing for shared station users

[dannymcc]

Summary

• Fixes fuel price history not being recorded when logging fuel at shared stations (station owned by another user)
• Fixes dashboard "Cheapest Fuel" section showing empty for users who don't own the stations
• Fixes "View All" cheapest fuel page showing "No price data available" despite fuel being logged

Root Cause

When fuel stations were made system-wide in #57, two places still had user-scoped checks:

1. app/routes/fuel.py:104 — Price history was only saved when station.user_id == current_user.id, so shared station users never had prices recorded
2. app/routes/main.py:113 — Dashboard query filtered FuelPriceHistory.user_id == current_user.id, hiding prices recorded by other users

Changes

• Removed ownership check in fuel logging — price history is now saved for any valid station
• Removed user_id filter from dashboard cheapest fuel query — all price data is now visible

Fixes #75

Test plan

• Log fuel at a station created by another user — verify price history is recorded
• Check dashboard "Cheapest Fuel" section shows data for shared station prices
• Click "View All" — verify prices appear for all stations
• Verify existing single-user setups still work correctly