fix: scope selector UX, ARIA, and code quality improvements

- removeScope preserves graph state when items remain (only resets on empty)
- Extract shared CheckIcon, CloseIcon, Spinner, ScopeListItem components
- Unify three handleSelect* into single handleToggle
- Fix shared mutable Set in reset state (factory function)
- Add MAX_SCOPE_ITEMS UI feedback (disabled button, warning banner)
- Correct ARIA: dialog > tablist + listbox > option structure
- Event listeners only attached when dropdown is open
- isSelected uses Set lookup via useMemo instead of O(n) scan
- Clear user search only on add, not on deselect

src/locales/en.json

@@ -1309,7 +1309,8 @@
         "noResults": "Nothing found",
         "removeItem": "Remove {{label}}",
         "clearAll": "Clear all selections",
-        "addScope": "Add scope"
+        "addScope": "Add scope",
+        "maxReached": "Maximum {{max}} items"
       },
       "loading": "Loading graph...",
       "error": "Failed to load network data",