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/components/admin/SystemSettingsClient.tsx

@@ -118,6 +118,10 @@ export function SystemSettingsClient() {
   const [vacationDefaultDays, setVacationDefaultDays] = useState(28);
   const [vacationSaved, setVacationSaved] = useState(false);
 
+  // Timeline
+  const [undoMaxSteps, setUndoMaxSteps] = useState(50);
+  const [timelineSaved, setTimelineSaved] = useState(false);
+
   const { data: settings, isLoading } = trpc.settings.getSystemSettings.useQuery(undefined, {
     staleTime: 0,
   });
@@ -152,6 +156,8 @@ export function SystemSettingsClient() {
       setAnonymizationSeed("");
       // Vacation
       setVacationDefaultDays(settings.vacationDefaultDays ?? 28);
+      // Timeline
+      setUndoMaxSteps(settings.timelineUndoMaxSteps ?? 50);
     }
   }, [settings]);
 
@@ -227,6 +233,13 @@ export function SystemSettingsClient() {
     },
   });
 
+  const saveTimelineMutation = trpc.settings.updateSystemSettings.useMutation({
+    onSuccess: () => {
+      setTimelineSaved(true);
+      setTimeout(() => setTimelineSaved(false), 3000);
+    },
+  });
+
   function handleSaveSmtp() {
     saveSmtpMutation.mutate({
       smtpHost: smtpHost || undefined,
@@ -242,6 +255,10 @@ export function SystemSettingsClient() {
     saveVacationMutation.mutate({ vacationDefaultDays });
   }
 
+  function handleSaveTimeline() {
+    saveTimelineMutation.mutate({ timelineUndoMaxSteps: undoMaxSteps });
+  }
+
   function handleSaveAnonymization() {
     saveAnonymizationMutation.mutate({
       anonymizationEnabled,
@@ -1226,6 +1243,46 @@ export function SystemSettingsClient() {
         </div>
       </div>
 
+      <div className={PANEL_CLASS}>
+        <div>
+          <h2 className="text-base font-semibold text-gray-900 dark:text-gray-100 flex items-center">
+            Timeline <InfoTooltip content="Settings for the timeline view, including undo history depth." />
+          </h2>
+          <p className="text-xs text-gray-500 dark:text-gray-400 mt-1">
+            Configure timeline behavior and undo/redo history.
+          </p>
+        </div>
+
+        <div className="max-w-xs">
+          <label className={LABEL_CLASS}>Undo History Depth</label>
+          <input
+            type="number"
+            className={INPUT_CLASS}
+            value={undoMaxSteps}
+            onChange={(e) => setUndoMaxSteps(parseInt(e.target.value, 10) || 50)}
+            min={1}
+            max={200}
+          />
+          <p className="text-xs text-gray-400 dark:text-gray-500 mt-1">
+            Maximum number of undo steps for timeline operations (single moves and batch shifts). Default: 50.
+          </p>
+        </div>
+
+        <div className="flex items-center gap-3">
+          <button
+            type="button"
+            onClick={handleSaveTimeline}
+            disabled={saveTimelineMutation.isPending}
+            className={PRIMARY_BUTTON_CLASS}
+          >
+            {saveTimelineMutation.isPending ? "Saving…" : "Save Timeline Settings"}
+          </button>
+          {timelineSaved && (
+            <span className="text-sm text-green-600 dark:text-green-400 font-medium">Saved!</span>
+          )}
+        </div>
+      </div>
+
       <div className={PANEL_CLASS}>
         <div>
           <h2 className="text-base font-semibold text-gray-900 dark:text-gray-100 flex items-center">