Prioritize editor rendering over thumbnail refreshes

[reville]

Summary

• keep the existing 300 ms sidecar autosave, but coalesce adjusted-thumbnail refreshes until the editor has been idle for 1.5 seconds
• prioritize queued GPU work as interactive editor, selected thumbnail, visible thumbnail, background work, then export
• register editor priority before preview preparation so queued thumbnail work cannot jump ahead
• suppress stale thumbnail events across edits, batch operations, cancellations, and folder/album navigation
• flush the pending metadata save before cancelling old-folder thumbnail work

Validation

• cargo test --manifest-path src-tauri/Cargo.toml (10 passed)
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets --all-features -- -D warnings
• npm run build
• npx prettier --check src/hooks/useAppNavigation.ts src/hooks/useEditorActions.ts

The repository-wide TypeScript and ESLint checks still surface pre-existing errors on main; neither reports a new error on the changed lines.