feat(dsh): add Invoice Downloader plugin

[EthanYoQ]

Summary

• Adds the installable @ethanyoq/dsh-invoice-downloader package with a right-side DeepSeek Harness panel for local IMAP invoice download, OCR, archive, and Excel summaries.
• Adds native output-directory selection, Host-side manual-path validation, profile-state Python runtime installation, and Windows x64/macOS Apple Silicon support.
• Adds source-file allowlisting, picker lifecycle tests, built-package validation, a macOS smoke workflow, and Chinese/English installation documentation.

Demo

Recorded interface commit: 4ec6498510528fa452a4ddab4f7bdaad7f1fbcd4
Current PR head: 11e4d8e502db3f39570e7ad6147a85a1453b949b

The operating-system directory chooser was not automated by the browser; the recording shows its panel button and uses an isolated absolute output directory. The run used a real IMAP connection and a redacted one-day scan on 2026-06-11. A real model round was not independently verified in the recording.

Checks

• npm run clean && npm test (19 tests)
• npm run test:package
• npm pack --json
• Windows runtime health check (7 checks)
• DSH Web: right-side panel, engine health, IMAP connection, output-directory flow, and redacted scan completion
• macOS macos-14: isolated tarball install, profile-state runtime health check, and DSH client-bundle load