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/dto/create-account.dto.ts

@@ -36,4 +36,8 @@ export class CreateAccountDto {
   @ApiProperty({ required: false, default: 0 })
   @IsOptional()
   initialBalance?: number;
+
+  @ApiProperty({ required: false, description: 'Annual interest rate as a percentage' })
+  @IsOptional()
+  interestRate?: number;
 }