docs: improve GitHub and npm discoverability

[CoderLambert]

Summary

• sharpen the GitHub and npm positioning around a browser-only React viewer for 20+ formats
• make English the primary repository README while preserving the full Chinese guide
• improve npm description, homepage, search keywords, onboarding, and troubleshooting
• add repository-level LGPL licensing, contribution/security guidance, issue templates, screenshots, and social-preview sources
• add a patch Changeset so the npm metadata and README ship through the existing release workflow

User impact

Developers can identify FileVista's supported formats and privacy model from search results, reach the live demo immediately, choose the correct base/full/plugin setup, and resolve common integration problems more quickly.

Validation

• pnpm run lint
• pnpm typecheck
• pnpm test — 16 files, 162 tests passed
• pnpm run build
• npm pack --dry-run --json
• git diff --check
• GitHub About, Homepage, Topics, Social Preview, and private vulnerability reporting verified