fix(export): make cancellation cooperative

[VailElla]

Description

Fix export cancellation so it cooperatively stops dispatching work and lets already-started workers exit at safe checkpoints. The active export remains registered through worker cleanup and terminal event emission, preventing stale cancellation or completion events from racing with a later export.

Type of Change

• Bug fix
• New feature
• Breaking change
• Performance improvement
• Code refactoring
• Documentation update
• UI/UX improvement
• Build/CI or Dependency update

Changes Made

• Replaced abort-only coordinator cancellation with a per-export atomic cancellation token.
• Preserved cancellation requested during startup and made repeated cancellation requests idempotent.
• Stopped dispatching new work after cancellation and added checkpoints around loading, rendering, mask/LUT processing, metadata writes, and file writes.
• Kept the active export slot through worker cleanup and terminal event serialization, preventing stale task events from affecting a later export.
• Added a visible Cancelling UI state; export settings stay locked while workers clean up, and a failed cancel invoke restores the retryable exporting state.
• Added the new status translation to all supported locales and kept the locale files extractor-sorted.
• Routed batch failures through export-error so the UI does not remain stuck at the final progress count.
• Added five unit tests covering startup cancellation, duplicate cancellation, overlapping exports, completion races, and stale task cleanup.
• Registered the legacy objc macro's feature = "cargo-clippy" condition so full clippy with -D warnings passes locally.

Screenshots/Videos

N/A — the UI change is a transient cancellation status rather than a persistent layout change.

Testing

• I have tested these changes locally and confirmed that they work as expected without issues

Test Configuration:

• OS: macOS 26.5.2
• Hardware: Apple M4 Pro, 14 logical cores, 48 GiB RAM

Automated:

• cargo +1.96.1 fmt --manifest-path src-tauri/Cargo.toml --check — passed
• cargo +1.96.1 check --manifest-path src-tauri/Cargo.toml — passed
• cargo +1.96.1 test --manifest-path src-tauri/Cargo.toml export_processing::tests:: -- --nocapture — 5 passed, 0 failed
• cargo +1.96.1 test --manifest-path src-tauri/Cargo.toml -- --nocapture — 5 passed, 0 failed
• cargo +1.96.1 clippy --manifest-path src-tauri/Cargo.toml --all-targets --all-features -- -D warnings — passed
• npm run build — passed
• npm run i18n:check — passed; 952 plural resolutions validated across 12 locales
• Changed-file Prettier check — passed
• Targeted ESLint results match the current main baseline exactly; no new findings
• git diff --check — passed

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors

Additional Notes

• Rebased onto main at 41d92c8f; the old upstream export-handle guard was removed because this PR's token lifecycle now owns registration, cancellation, cleanup, and terminal-event serialization.
• No RAW decoding or image-processing algorithm changes are included.
• A new export can start only after the previous workers have been cleaned up and its terminal event has been emitted.

AI Disclaimer:

Please state the involvement of AI in this PR:

• This PR is entirely AI-generated
• This PR is AI-generated but guided by a human
• This PR was handwritten with AI assistance (spell check, logic suggestions, error resolving)
• This PR contains only blood, sweat, and coffee (AI-free)