* feat: add copy-link button to share viewer messages
Implements the feature requested in #437:
- Add a small link icon button that appears on hovering over user/assistant
messages in the share viewer
- Clicking the button copies a shareable URL to clipboard with visual feedback
- URL format: base?gistId&leafId=<active-leaf>&targetId=<message-id>
- When loading a URL with leafId and targetId params:
- Navigate to the specified leaf node
- Scroll to and briefly highlight the target message
This enables users to share links to specific messages within a session.
* fix: preserve gist ID format and add clipboard fallback
- Fix URL format to produce ?gistId&leafId=... instead of ?gistId=&leafId=...
(preserves the bare key format expected by the backend)
- Add execCommand fallback for clipboard copy on HTTP contexts where
navigator.clipboard is unavailable
cf10e9592a