This commit enables users on custom Personal Data Servers (PDS) to sign in, addressing the limitation mentioned in the README warning.

Key features:
- Added optional PDS Server field to login form
- Comprehensive URL validation with security checks (HTTPS required except localhost)
- Auto-normalization of PDS URLs (adds https:// if missing)
- Enhanced error handling with user-friendly messages
- Progress indicators during authentication (spinner in sign-in button)
- Client-side validation for handle format and PDS URL format
- Support for both bsky.social (default) and custom PDS servers

Technical changes:
- popup/popup.html: Added PDS server input field with helpful hints
- popup/popup.js: Added validation, normalization, and progress UI
- popup/styles.css: Added button spinner animation
- lib/atproto.js: Updated createSession to accept optional service parameter
- background/background.js: Updated authenticate to pass service to API
- README.md: Removed warning, documented custom PDS support

Security features:
- HTTPS enforcement for production servers
- HTTP allowed only for localhost/local networks
- Validation prevents malformed URLs
- Clear error messaging for network/auth failures

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>