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>

src/config/env.ts

@@ -4,10 +4,11 @@ dotenv.config();
 // Set timezone for Date operations — all attendance/time records are in Czech local time
 process.env.TZ = process.env.TZ || "Europe/Prague";
 
-// Override Date.toJSON to serialize as local time instead of UTC
-// MySQL DATETIME stores local time, Prisma creates Date objects,
-// JSON.stringify calls toJSON() which defaults to toISOString() (UTC with Z suffix).
-// This causes times to shift by timezone offset on the frontend.
+// Override Date.toJSON so JSON.stringify outputs local time (Europe/Prague).
+// Prisma stores UTC in MySQL DATETIME columns. When reading, it creates
+// JS Date objects with correct UTC internals. The default toJSON() calls
+// toISOString() which returns UTC — this override uses local getters instead,
+// so the frontend always receives times in the user's timezone.
 Date.prototype.toJSON = function () {
   const y = this.getFullYear();
   const m = String(this.getMonth() + 1).padStart(2, "0");
@@ -53,6 +54,8 @@ export const config = {
 
   nas: {
     path: process.env.NAS_PATH || "Z:/02_PROJEKTY",
+    financialsPath: process.env.NAS_FINANCIALS_PATH || "",
+    offersPath: process.env.NAS_OFFERS_PATH || "",
     maxUploadSize: parseInt(process.env.MAX_UPLOAD_SIZE || "52428800", 10),
   },