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/main.py

@@ -218,6 +218,13 @@ def _migrate():
         except Exception:
             pass
 
+        try:
+            conn.execute(text("ALTER TABLE user_settings ADD COLUMN share_calendar_icon VARCHAR(16)"))
+            conn.commit()
+            logging.info("Migration: added share_calendar_icon to user_settings")
+        except Exception:
+            pass
+
 _migrate()
 
 app = FastAPI(title="Calendarr", docs_url=None, redoc_url=None)