Quality-of-life enhancements: CSV import/export, opening balances, interest rates, mobile UX

- CSV import/export for Units, Projects, and Vendors with match-on-name/number upsert
- Cash Flow report toggle for Cash Only vs Cash + Investments
- Per-account and bulk opening balance setting with as-of date
- Interest rate field on normal accounts with estimated monthly/annual interest display
- Mobile sidebar auto-close on navigation
- Shared CSV parsing/export utility extracted to frontend/src/utils/csv.ts

DB migration needed for existing tenants:
  ALTER TABLE accounts ADD COLUMN IF NOT EXISTS interest_rate DECIMAL(6,4);

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

backend/src/modules/accounts/accounts.controller.ts

@@ -32,6 +32,23 @@ export class AccountsController {
     return this.accountsService.setPrimary(id);
   }
 
+  @Post('bulk-opening-balances')
+  @ApiOperation({ summary: 'Set opening balances for multiple accounts' })
+  bulkSetOpeningBalances(
+    @Body() dto: { asOfDate: string; entries: { accountId: string; targetBalance: number }[] },
+  ) {
+    return this.accountsService.bulkSetOpeningBalances(dto);
+  }
+
+  @Post(':id/opening-balance')
+  @ApiOperation({ summary: 'Set opening balance for an account at a specific date' })
+  setOpeningBalance(
+    @Param('id') id: string,
+    @Body() dto: { targetBalance: number; asOfDate: string; memo?: string },
+  ) {
+    return this.accountsService.setOpeningBalance(id, dto);
+  }
+
   @Post(':id/adjust-balance')
   @ApiOperation({ summary: 'Adjust account balance to a target amount' })
   adjustBalance(