feat: Sprint 4 — scenario planner, report builder, comments, dashboard widgets

What-If Scenario Planner (G5):
- New /projects/[id]/scenario page with side-by-side baseline vs scenario
- simulate mutation: pure cost/hours/headcount/utilization computation
- apply mutation: creates real PROPOSED assignments from scenario
- Impact cards: cost delta, hours delta, headcount, skill coverage %
- Per-resource utilization impact table with over-allocation warnings
- "What-If" button added to project detail page

Custom Report Builder (G7):
- New /reports/builder page with full config panel
- Entity selector (resource/project/assignment), column picker, filter builder
- Dynamic Prisma query with eq/neq/gt/lt/contains/in operators
- Sortable results table with pagination (50/page)
- CSV export via exportReport mutation
- Sidebar nav link under Analytics

Collaboration Layer (G8):
- Comment model in Prisma (entityType/entityId, replies, @mentions, resolved)
- comment router: list, count, create, resolve, delete
- @mention parsing with notification creation + SSE delivery
- CommentInput with @mention autocomplete (arrow nav, Enter/Tab confirm)
- CommentThread with avatar, timestamp, reply, resolve, delete
- Integrated as "Comments" tab in estimate workspace with count badge

Dashboard Widgets:
- BudgetForecastWidget: progress bars per project, burn rate, exhaustion date
- SkillGapWidget: supply vs demand per skill, shortage/surplus indicators
- ProjectHealthWidget: 3-dimension health circles + composite score
- 3 new application use-cases + dashboard router queries
- All registered in widget-registry with lazy imports

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

apps/web/src/components/layout/AppShell.tsx

@@ -61,6 +61,9 @@ function MarketplaceIcon() {
 function ChargeabilityIcon() {
   return <svg className="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={1.8} d="M5 17l4-4 3 3 7-8M19 19H5V5" /></svg>;
 }
+function ReportBuilderIcon() {
+  return <svg className="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={1.8} d="M9 17v-2m3 2v-4m3 4v-6m2 10H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z" /></svg>;
+}
 function GraphIcon() {
   return <svg className="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><circle cx="6" cy="6" r="2.5" strokeWidth={1.8} /><circle cx="18" cy="6" r="2.5" strokeWidth={1.8} /><circle cx="12" cy="18" r="2.5" strokeWidth={1.8} /><path strokeLinecap="round" strokeWidth={1.8} d="M8.5 7.5l2 7M15.5 7.5l-2 7M8.5 6h7" /></svg>;
 }
@@ -141,6 +144,7 @@ const navSections: NavSection[] = [
       { href: "/analytics/skills", label: "Skills Analytics", icon: <SkillsIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER", "VIEWER"] },
       { href: "/analytics/skill-marketplace", label: "Skill Marketplace", icon: <MarketplaceIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
       { href: "/reports/chargeability", label: "Chargeability", icon: <ChargeabilityIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
+      { href: "/reports/builder", label: "Report Builder", icon: <ReportBuilderIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
       { href: "/analytics/computation-graph", label: "Computation Graph", icon: <GraphIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
     ],
   },