feat(desktop): add scanned request exports

[kev1n77]

Summary

• add versioned Minimal, Reproducible, and Forensic export DTOs and generated TypeScript bindings
• scan every exported string for Anthropic/OpenAI/Google keys, bearer tokens, JWTs, and private keys, replacing matches with stable placeholders and JSON Pointer evidence
• require a full backend-generated preview, then recompute the same export and verify SHA-256 before writing
• save through the native Tauri dialog and only create new absolute JSON files with private Unix permissions
• add an accessible export dialog with profile switching, exact JSON preview, redaction details, save status, Escape handling, and inert background
• update SEC-002 progress, security controls, and threat-model evidence

Verification

• cargo fmt --all -- --check
• cargo fmt --manifest-path apps/desktop/src-tauri/Cargo.toml -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml (13 unit + 1 watchdog integration)
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check (21 tests)
• npm run desktop:build
• browser QA at default viewport and 960x620: no overflow or overlap, independently scrolling preview, Forensic profile exposes Raw, no console errors