Add selection validation and improve tooltip component

- Add validation system to prevent contradicting conditions:
  - Same entity in include/exclude detection
  - Parent-child conflict detection for tree entities
  - Redundant selection prevention
  - Toast notifications for validation errors

- Fix entity icons (employees: briefcase, taxes: calculator)

- Improve tooltip component:
  - Use Material Icons instead of broken FA4 icons
  - Fix positioning using getBoundingClientRect for viewport coords
  - Add click-to-pin functionality with close button
  - Pinned tooltips show X icon and close button in corner

- Add lightweight test suite (31 tests) for validation logic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sources/scss/main.scss

@@ -29,3 +29,4 @@
 @use 'components/method-dropdown';
 @use 'components/tooltip';
 @use 'components/tree';
+@use 'components/validation';