Rename project from LLM-Shield to PasteGuard (#5)

Rename project from LLM-Shield to PasteGuard (#5)

After positive community response (100+ GitHub stars), committing fully
to this project. The name PasteGuard better describes the core function:
"guard what you paste" before sending to LLMs.

Changes:
- Update all branding: package.json, README, CONTRIBUTING
- Rename response headers: X-LLM-Shield-* → X-PasteGuard-*
- Update dashboard UI and page title
- Update /info endpoint metadata
- Update startup banner
- Update config files and defaults
- Update CI workflow docker tag
- Regenerate bun.lock with new package name

Domain pasteguard.com secured. Old GitHub links redirect automatically.