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/database/tenant-schema.service.ts

@@ -45,6 +45,7 @@ export class TenantSchemaService {
         is_active BOOLEAN DEFAULT TRUE,
         is_system BOOLEAN DEFAULT FALSE,
         is_primary BOOLEAN DEFAULT FALSE,
+        interest_rate DECIMAL(6,4),
         balance DECIMAL(15,2) DEFAULT 0.00,
         created_at TIMESTAMPTZ DEFAULT NOW(),
         updated_at TIMESTAMPTZ DEFAULT NOW(),