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/units/units.controller.ts

@@ -1,5 +1,6 @@
-import { Controller, Get, Post, Put, Delete, Body, Param, UseGuards } from '@nestjs/common';
+import { Controller, Get, Post, Put, Delete, Body, Param, Res, UseGuards } from '@nestjs/common';
 import { ApiTags, ApiBearerAuth } from '@nestjs/swagger';
+import { Response } from 'express';
 import { JwtAuthGuard } from '../auth/guards/jwt-auth.guard';
 import { UnitsService } from './units.service';
 
@@ -13,9 +14,19 @@ export class UnitsController {
   @Get()
   findAll() { return this.unitsService.findAll(); }
 
+  @Get('export')
+  async exportCSV(@Res() res: Response) {
+    const csv = await this.unitsService.exportCSV();
+    res.set({ 'Content-Type': 'text/csv', 'Content-Disposition': 'attachment; filename="units.csv"' });
+    res.send(csv);
+  }
+
   @Get(':id')
   findOne(@Param('id') id: string) { return this.unitsService.findOne(id); }
 
+  @Post('import')
+  importCSV(@Body() rows: any[]) { return this.unitsService.importCSV(rows); }
+
   @Post()
   create(@Body() dto: any) { return this.unitsService.create(dto); }