feat(epub-reader): add support for custom themes

[xcashy]

🚀 Pull Request

📝 Description

With this PR users can create, edit, and delete custom color themes for the EPUB reader

🧪 Testing Strategy

• Open Settings → Reader Preferences → EPUB
• Click the "+" button in the theme selector to create a new custom theme
• Verify the color picker and hex input work correctly for all 6 color fields
• Verify the live preview updates in real-time
• Save the theme and confirm it appears in the theme selector with a user badge
• Select the custom theme and open an EPUB book to verify it applies correctly
• Edit the custom theme and verify changes persist
• Delete the custom theme and verify the selection reverts to default
• Verify custom themes sync across browser sessions

📸 Visual Changes (if applicable)

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⚠️ Required Pre-Submission Checklist

Please Read - This Checklist is Mandatory

Important Notice: We've experienced several production bugs recently due to incomplete pre-submission checks. To maintain code quality and prevent issues from reaching production, we're enforcing stricter adherence to this checklist.

All checkboxes below must be completed before requesting review. PRs that haven't completed these requirements will be sent back for completion.

Mandatory Requirements (please check ALL boxes):

• Code adheres to project style guidelines and conventions
• Branch synchronized with latest develop branch (please resolve any merge conflicts)
• 🚨 CRITICAL: Automated unit tests added/updated to cover changes (MANDATORY for ALL Spring Boot backend and Angular frontend changes - this is non-negotiable)
• 🚨 CRITICAL: All tests pass locally (run ./gradlew test for Spring Boot backend, and ng test for Angular frontend - NO EXCEPTIONS)
• 🚨 CRITICAL: Manual testing completed in local development environment (verify your changes work AND no existing functionality is broken - test related features thoroughly)
• Flyway migration versioning follows correct sequence (if database schema was modified)
• Documentation PR submitted to booklore-docs (required for features or enhancements that introduce user-facing or visual changes)

Why This Matters:

Recent production incidents have been traced back to:

• Incomplete testing coverage (especially backend)
• Merge conflicts not resolved before merge
• Missing documentation for new features

Backend changes without tests will not be accepted. By completing this checklist thoroughly, you're helping maintain the quality and stability of Booklore for all users.

Note to Reviewers: Please verify the checklist is complete before beginning your review. If items are unchecked, kindly ask the contributor to complete them first.

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💬 Additional Context (optional)