Bug & tweak sprint: fix financial calculations, add quarterly report, enhance dashboard

- Fix Accounts page: include investment accounts in Est. Monthly Interest calc,
  add Fund column to investment table, split summary cards into Operating/Reserve
- Fix Cash Flow: ending balance now respects includeInvestments toggle
- Fix Budget Manager: separate operating/reserve income in summary cards
- Fix Projects: default sort by planned_date instead of name
- Add Vendors: last_negotiated date field with migration, CSV import/export
- New Quarterly Financial Report: budget vs actuals, over-budget flagging, YTD
- Enhance Dashboard: separate Operating/Reserve fund cards, expanded Quick Stats
  with monthly interest, YTD interest earned, planned capital spend

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -74,6 +74,7 @@ const navSections = [
           { label: 'Aging Report', path: '/reports/aging' },
           { label: 'Sankey Diagram', path: '/reports/sankey' },
           { label: 'Year-End', path: '/reports/year-end' },
+          { label: 'Quarterly Financial', path: '/reports/quarterly' },
         ],
       },
     ],