fix: inherit settings for extension downloads

[vanloctech]

Summary

• preserve global download settings for browser extension requests
• keep extension-selected media type and quality as per-item overrides
• add regression coverage for global MKV, playlist, and subtitle snapshots

Verification

• bun run biome check --write .
• bun run tsc -b
• cargo check
• bun test ./tests/download-settings.test.ts