Cross-DE tray polish: plain-text tooltips, GNOME install docs, menu tweak
- Strip HTML from tooltips — use plain text for KDE/GNOME compatibility
- INSTALL.md: add GNOME AppIndicator extension instructions (step 6)
with distro-specific install commands and verification steps
- Move "open journal" to top level above settings/about submenus
- Update activity detection note (works on KDE too now)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>