feat(sidebar): add quick worktree session filtering

[lyx2145181]

Summary

• add compact All / 1 / 2 / … shortcuts below the existing worktree selector
• filter the conversation list to the selected checkout without another server request
• keep running and unread activity visible on each shortcut, including accessible labels
• treat same-project worktree changes as cwd context changes while leaving background runs untouched
• normalize browser-side worktree path comparisons across trailing separators and Windows path variants

The existing worktree dropdown remains responsible for branch/path details, creation, and removal.

Testing

• targeted sidebar, workspace-memory, agent-session, and project-group tests: 46 passed
• node_modules/.bin/tsc --noEmit
• npm run lint
• git diff --check upstream/main...HEAD