[FEAT] Custom in-app notifications for scheduled blocks #161

Is your feature request related to a problem? Please describe.

ClimbStack's primary goal is eliminating decision fatigue and preventing users from getting lost scrolling their phones during free time. While blocks can be scheduled in the planner, users who are distracted might miss the transition to their next block unless they rely on external calendar alerts.

Describe the solution you'd like

Add native local notifications for scheduled blocks via expo-notifications:

1. Settings Configuration:
   • Master toggle: Enable/disable block start reminders.
   • Lead time setting: When to trigger the notification (e.g., "At start", "5 minutes before", "10 minutes before", "15 minutes before").
   • Sound option: Normal sound or silent / vibration only.
2. Scheduling Engine:
   • Automatically schedule local notifications when blocks are added or applied from a template.
   • Cancel/reschedule notifications when blocks are deleted, edited, or cleared.
   • Clean notification content (e.g., "⏰ Starting soon: Study OSTEP").

Describe alternatives you've considered

Relying solely on Google Calendar synchronization notifications, but not all users sync with external calendars, and local-first offline notifications ensure immediate reliability.

Additional Context

• Distinct from [FEAT] Daily Reminder Notifications #11 (which handles the morning summary notification).
• Mentioned in ROADMAP.md backlog: "Native internal notifications/reminders for scheduled blocks (if Google Calendar isn't enough)".
• AI Generation Disclosure: This issue description was generated by an AI agent (Gemini 3.1 Pro).