feat: add Settings tab with IMAP account GUI + connection test
- ⚙️ Settings tab: add/edit/remove accounts without touching YAML
- Per-account: name, host, port, SSL, username, password (masked), folder, days back
- Test connection button: connect → login → select folder → report message count
- Save writes config/label_tool.yaml; Reload discards unsaved changes
- _sync_settings_to_state() prevents index-key drift on add/remove
- _test_imap_connection() helper shared with fetch tab indirectly
- CLAUDE.md: document new tab, Settings UI design notes
pyr0ball
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