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/routers/settings_router.py

@@ -31,6 +31,7 @@ class SettingsUpdate(BaseModel):
     group_visible_calendar_id: Optional[int] = None
     default_reminder_minutes: Optional[int] = None  # null = off
     default_event_duration_minutes: Optional[int] = None
+    share_calendar_icon: Optional[str] = None
 
 
 def _settings_dict(s: models.UserSettings) -> dict:
@@ -54,6 +55,7 @@ def _settings_dict(s: models.UserSettings) -> dict:
         "group_visible_calendar_id": s.group_visible_calendar_id,
         "default_reminder_minutes": s.default_reminder_minutes,
         "default_event_duration_minutes": s.default_event_duration_minutes or 60,
+        "share_calendar_icon": s.share_calendar_icon,
     }
 
 
@@ -96,7 +98,7 @@ def update_settings(
     # For these three override colours, an explicit null is meaningful
     # ("reset to default") and must be persisted as NULL. All other fields
     # keep the previous behaviour where a null/missing value is ignored.
-    NULLABLE_OVERRIDES = {"text_color", "line_color", "bg_color", "group_visible_calendar_id", "default_reminder_minutes", "default_event_duration_minutes"}
+    NULLABLE_OVERRIDES = {"text_color", "line_color", "bg_color", "group_visible_calendar_id", "default_reminder_minutes", "default_event_duration_minutes", "share_calendar_icon"}
     update_data = data.model_dump(exclude_unset=True)
     for field, value in update_data.items():
         if field in NULLABLE_OVERRIDES: