Add document storage (local filesystem) + fix dashboard outstanding KPI

- Local file storage via STORAGE_PATH env var (replaces DO Spaces)
- POST/GET/DELETE /api/cases/:caseId/documents + GET /api/documents/:docId/download
- useDocuments hook + upload/list/download/delete UI in CaseDetailPage
- GET /api/invoices/summary — live SUM of sent+overdue invoices
- Dashboard outstanding KPI wired to real DB value
- api.upload() for FormData, formatBytes() utility

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

.env.example

@@ -22,13 +22,11 @@ DATABASE_URL=postgresql://doadmin:password@db-postgresql-nyc1-xxxxx.b.db.ondigit
 DATABASE_CA_CERT_PATH=./certs/do-ca.crt
 
 # ─────────────────────────────────────────────
-# DigitalOcean Spaces (S3-compatible)
+# Local file storage
+# Absolute path where uploaded documents are stored (outside web root).
+# Production example: /var/www/vhosts/elegalsoftware.com/storage
 # ─────────────────────────────────────────────
-SPACES_ENDPOINT=https://nyc3.digitaloceanspaces.com
-SPACES_REGION=nyc3
-SPACES_BUCKET=lawdesk-uploads
-SPACES_ACCESS_KEY=
-SPACES_SECRET_KEY=
+STORAGE_PATH=./storage
 
 # ─────────────────────────────────────────────
 # Email (Resend)