Add comprehensive test infrastructure with real Anki data

[sysid]

Summary

• Implement comprehensive test infrastructure using real Anki collection data
• Add golden dataset with 15 diverse notes covering various content types
• Create test fixture system with automatic dataset refresh capability
• Expand test coverage from basic unit tests to full integration tests

Changes

Test Infrastructure

• Added TestCollection helper for managing temporary test collections
• Implemented fixture copying system to isolate test data
• Created shell script to refresh fixtures from golden dataset
• Added test note ID constants for consistent test references

Test Coverage

• AnkiRepository tests: Note retrieval, deletion, media directory access (8 tests)
• HtmlPresenter tests: Media path resolution, HTML processing, multi-image handling (5 tests)
• NoteDeleter tests: Note and card deletion, error handling (3 tests)
• NoteViewer tests: End-to-end note viewing with rendering (4 tests)

Test Data

• Golden dataset in data/testuser/ with 15 real Anki notes
• Test fixture in ankiview/tests/fixtures/test_collection/
• 4 media files (PNG images) for testing image path resolution
• Diverse content: data structures, algorithms, data science metrics, geographic concepts

Developer Experience

• New Makefile targets: refresh-test-fixture, test-verbose
• Media cleanup script for maintaining test data consistency
• Comprehensive fixture documentation in README files