Add optional reading goals, plans, streaks, and achievements

[mokhov]

Problem

Readers who want help building a habit cannot set goals or plan when to finish a book inside the reader.

Proposed behavior

Add fully optional daily, yearly, and finish-by-date plans with gentle reminders and opt-in achievements.

Acceptance criteria

• Goals can use minutes, books, progress, or a target completion date.
• Finish-by-date plans translate the remaining book into a clear daily pace.
• Streaks, achievements, coaching copy, and notifications are independently optional.
• Missing a goal does not use punitive language or block reading.
• All goal data remains private and works without an account.