[Feature] Add payment reminder before starting journey tracking

[mattekudacy]

Description

Display a friendly reminder to pay the fare before users start tracking their destination.

User Story

As a commuter, I want to be reminded to pay my fare before tracking starts, so I don't forget to tap my Beep card or pay the conductor.

Proposed Solution

Show a reminder dialog/bottom sheet after selecting destination

💳 Don't forget to pay!

Tap your Beep card or pay the conductor before settling in.

[ ] Don't show this again

[Start Tracking]

Acceptance Criteria

• Reminder appears after destination is selected
• User can dismiss and proceed to tracking
• "Don't show again" preference saved to SharedPreferences
• Can be re-enabled in settings