feat: export filtered requests as a secure batch

[kev1n77]

Summary

• export the current filtered request set as one versioned JSON document while preserving request order
• scan every request with the existing credential scrubber and revalidate SHA-256 before creating a new file
• reject empty, duplicate, missing, or over-limit batches and allow historical search results to load directly by ID
• reuse the export dialog for single and batch workflows, with native commands and a synthetic web fallback
• update SEC-002 progress and threat-model evidence without marking support bundles or the expanded secret corpus complete

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build
• local browser QA: filtered OpenAI requests open a two-request batch preview with no console warnings or errors