feat(ui): add elevation system for overlays and modals

- Add 3-level elevation CSS variables to themes.css for all theme variants
- elevation-1: dropdowns, pills (16% in dark mode)
- elevation-2: modals, overlays (20% in dark mode)
- elevation-3: context menus, tooltips (24% in dark mode)
- Update ContextMenu to use elevation-3
- Update Modal to use elevation-2 with theme-aware borders
- Update QuickEventOverlay to use elevation-2 with matching footer
- Update PillTimeRangeSelector dropdown to use elevation-1
- Update ConfirmationModal and FormModal to use theme variables
- Remove shadows from overlay components for cleaner look

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/shared-ui/src/organisms/ConfirmationModal.svelte

@@ -162,8 +162,8 @@
 				onclick={onClose}
 				disabled={loading}
 				class="w-full flex items-center justify-center gap-2 px-4 py-3 rounded-xl font-semibold text-sm
-					   bg-black/5 dark:bg-white/10 text-foreground
-					   hover:bg-black/10 dark:hover:bg-white/20 hover:shadow-md
+					   bg-foreground/5 text-foreground
+					   hover:bg-foreground/10 hover:shadow-md
 					   transition-all duration-200 hover:-translate-y-0.5 active:translate-y-0
 					   disabled:opacity-50 disabled:cursor-not-allowed disabled:hover:translate-y-0"
 			>