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>

backend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "hoa-ledgeriq-backend",
-  "version": "2026.3.11",
+  "version": "2026.03.16",
   "description": "HOA LedgerIQ - Backend API",
   "private": true,
   "scripts": {