feat: timeline multi-select, demand popover, resource hover card, merged tooltips, dark mode fixes

Major timeline enhancements:
- Right-click drag multi-selection with floating action bar (batch delete/assign)
- DemandPopover for demand strip details (replaces broken "Loading" modal)
- ResourceHoverCard on name hover showing skills, rates, role, chapter
- Merged heatmap+vacation tooltips into unified TimelineTooltip component
- Fixed overbooking blink animation (date normalization, z-index ordering)
- Fixed dark mode sticky column bleed-through in project view
- System roles admin page, notification task management, performance review docs

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/hooks/useTimelineLayout.tsx

@@ -44,10 +44,10 @@ export function useTimelineLayout(
         key={i}
         className={clsx(
           "absolute top-0 bottom-0 border-r",
-          isToday ? "border-brand-300 border-r-2" :
-          isSaturday ? "border-amber-200 bg-amber-50/40" :
-          isSunday ? "border-gray-200 bg-gray-100/60" :
-          "border-gray-100",
+          isToday ? "border-brand-300 dark:border-brand-700 border-r-2" :
+          isSaturday ? "border-amber-200 dark:border-amber-800 bg-amber-50/40 dark:bg-amber-950/20" :
+          isSunday ? "border-gray-200 dark:border-gray-700 bg-gray-100/60 dark:bg-gray-800/40" :
+          "border-gray-100 dark:border-gray-800",
         )}
         style={{ left: i * CELL_WIDTH, width: CELL_WIDTH }}
       />