feat: calendar rename for all sources, group icon in sidebar, configurable share icon

- Rename: Add google and homeassistant branches to the dblclick-rename
  save() handler; backend already accepted name updates for both sources
- Group icon: Pass groupIconForLocalCal(cal.id) into sidebar entries so
  the group's custom icon (home/heart/work/…) shows instead of always
  defaulting to the people icon
- Share icon: New share_calendar_icon field in UserSettings (+ migration)
  replaces the hardcoded SHARE_ICON SVG; users pick from GROUP_ICON_KEYS
  in Settings → Darstellung via a new icon-picker row

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/models.py

@@ -108,6 +108,8 @@ class UserSettings(Base):
     default_reminder_minutes = Column(Integer, nullable=True)
     # Default duration (in minutes) applied to a newly created event's end time.
     default_event_duration_minutes = Column(Integer, default=60)
+    # Icon key (from GROUP_ICON_KEYS) shown next to calendars this user shares with groups.
+    share_calendar_icon = Column(String(16), nullable=True)
 
     user = relationship("User", back_populates="settings")