feat(app): add staged book imports

[darwin67]

Summary

• discover selected files and folders before copying or linking anything
• group exact normalized filename variants and surface existing or repeated content
• let readers review formats, duplicates, storage behavior, and selected paths before confirmation
• cancel abandoned scans, reject stale picker results, and verify reviewed fingerprints at import time
• skip non-regular files, handle symlink targets consistently, and normalize Unicode grouping keys
• label candidate controls with distinguishable paths for accessibility

Regression coverage

• non-regular files and symlink extension mismatches
• stale picker and discovery completions
• deterministic failure ordering
• cancellation before and during fingerprinting
• files changed between review and import
• preserving explicit duplicate selections
• nested and individual candidate labels
• Unicode canonical equivalence and full case folding

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace