feat: NAS storage for invoices/offers, code cleanup, date/time fixes

- NAS storage for created invoices (PDF via puppeteer), received invoices,
  and offers with auto-save on create/edit
- Deterministic file paths derived from DB fields (no file_path column needed)
- Separate NAS mount points: NAS_FINANCIALS_PATH, NAS_OFFERS_PATH
- Invoice language field (cs/en) stored per invoice, replaces lang modal
- Invoices list filtered by month/year matching KPI card selection
- Centralized date helpers (src/utils/date.ts) replacing all .toISOString()
  calls that returned UTC instead of local time
- Attendance project switching uses exact time (not rounded)
- Comment cleanup: removed ~100 unnecessary/Czech comments
- Removed as-any casts in orders and attendance
- Prisma migrations: add invoice language, drop received_invoices BLOB columns

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

prisma/schema.prisma

@@ -167,6 +167,7 @@ model invoices {
   paid_date       DateTime?       @db.Date
   issued_by       String?         @db.VarChar(255)
   billing_text    String?         @db.VarChar(500)
+  language        String?         @default("cs") @db.VarChar(5)
   notes           String?         @db.Text
   internal_notes  String?         @db.Text
   created_at      DateTime?       @default(now()) @db.DateTime(0)
@@ -410,7 +411,6 @@ model received_invoices {
   due_date       DateTime?                @db.Date
   paid_date      DateTime?                @db.Date
   status         received_invoices_status @default(unpaid)
-  file_data      Bytes?                   @db.MediumBlob
   file_name      String?                  @db.VarChar(255)
   file_mime      String?                  @db.VarChar(100)
   file_size      Int?                     @db.UnsignedInt