fix: preserve partial download integrity

[amanharshx]

Summary

• Prevent annotation/image mismatches after partial download failures.
• Retry transient image download failures and report omitted records clearly.

Motivation

Expired or unavailable image URLs could produce successful ZIPs containing annotations for images missing from archive.

Key Changes

• Filter image records before conversion while preserving labels-only mode.
• Retry transport, body-read, 408, 429, and 5xx failures up to three attempts with bounded backoff.
• Preserve SSRF validation, redirect blocking, concurrency limit, and response-size cap.
• Report omitted annotations, missing URLs, and possible expired signed URLs.
• Hard-fail when no requested images can be archived.
• Testing: cargo test (89), cargo clippy --all-targets -- -D warnings, cargo fmt --check, bun run test (8), bun run build, and 20/20 normalized export fingerprints.
• Risk: partial failures now intentionally remove affected images and annotations.
• Sanity check: one failed download among two inputs -> expected one consistent image/annotation pair -> actual output contains only successful pair.

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Codecov Report

❌ Patch coverage is 89.63134% with 45 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src-tauri/src/downloader.rs	91.77%	25 Missing ⚠️
src-tauri/src/lib.rs	85.84%	16 Missing ⚠️
src/components/converter-screen.tsx	76.47%	4 Missing ⚠️

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