Split asset date group into (temp) generic and selection-aware components

• Rename asset-date-group.svelte to asset-date-group-comp.svelte as generic base
• Create asset-date-group-selection-aware.svelte wrapper for selection logic
• Extract selection-specific callbacks into separate handlers
• Update onDateGroupSelect to onDayGroupSelect with DayGroup parameter

.vscode/settings.json

@@ -50,7 +50,7 @@
     "editor.formatOnSave": true,
     "editor.tabSize": 2
   },
-  "cSpell.words": ["immich"],
+  "cSpell.words": ["immich", "intersectable", "intersectables"],
   "editor.formatOnSave": true,
   "eslint.validate": ["javascript", "svelte"],
   "explorer.fileNesting.enabled": true,