feat(login): improve login UX with explainer and simpler labels (#213)
* feat(login): improve login UX with explainer and simpler labels
- Simplify "Handle or DID" label to just "Handle" (DID/URI input still
works silently)
- Remove asterisk from sole required field
- Remove technical helper text about DID/URI formats
- Add collapsible "What is an AT Protocol identity?" explainer that
educates new users without cluttering the default view
- Update placeholder to alice.bsky.social for broader recognition
Inspired by npmx.dev's onboarding pattern.
* test(login): update tests for new placeholder and explainer links
- Update placeholder assertion from jay.bsky.team to alice.bsky.social
- Handle multiple Bluesky links (explainer + PDS section) in PDS host test
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