chore: reorganize sidebar navigation and bump version to 2026.03.16

Remove the Planning section. Move Projects and Capital Planning (as
sub-item) into Board Planning. Move Investment Planning with Investment
Scenarios as sub-item into Board Planning. Move Vendors into new Board
Reference section. Board Planning order: Budget Planning, Projects >
Capital Planning, Assessment Scenarios, Investment Planning > Investment
Scenarios, Compare Scenarios. Sidebar now supports parent items with
their own route plus nested children.

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

frontend/src/pages/investment-planning/InvestmentPlanningPage.tsx

@@ -917,7 +917,6 @@ export function InvestmentPlanningPage() {
             description="Purchase date for the investment(s). Maturity dates are calculated automatically from term length."
             value={investmentStartDate}
             onChange={setInvestmentStartDate}
-            clearable
           />
 
           {investmentScenarios && investmentScenarios.length > 0 && (