perf(app): improve responsiveness across the app

[darwin67]

Summary

• move document opening, EPUB image decoding, CBZ dimension reads, bookmark writes, and interactive searches off or away from the UI hot path
• stage EPUB pagination around the current chapter, cache reusable reader/library work, and bound nearby image decoding and continuous measurement costs
• add delayed neutral loading previews with book covers while preventing stale-content flicker
• render PDF covers directly at thumbnail resolution, track EPUB caption offsets in linear time, and reuse table placements between width and geometry calculations
• fix SVG-wrapped EPUB covers and pin paginated EPUB page numbers to a consistent footer baseline
• add EPUB latency budgets, benchmark validation, and regression coverage for staged pagination, resource limits, and the new local optimizations
• report per-RFD task numbers in make check-rfds

Verification

• cargo test -p shosai-core
• cargo test -p shosai-app (302 tests)
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• make test-scripts
• EPUB page-turn benchmark thresholds on Linux and macOS

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