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/utils/date.ts

@@ -0,0 +1,40 @@
+/**
+ * Centralized date/time helpers.
+ *
+ * Prisma stores DateTime as UTC in MySQL DATETIME columns.
+ * The Date.toJSON override in config/env.ts serializes using local getters,
+ * so the frontend always receives local time. These helpers ensure
+ * consistent local-time formatting whenever we need a string outside
+ * of JSON serialization (e.g., building lookup keys, shift_date strings).
+ */
+
+/** YYYY-MM-DD in local time */
+export function localDateStr(d: Date): string {
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
+}
+
+/** YYYY-MM in local time */
+export function localMonthStr(d: Date): string {
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
+}
+
+/** HH:MM in local time */
+export function localTimeStr(d: Date): string {
+  return `${String(d.getHours()).padStart(2, "0")}:${String(d.getMinutes()).padStart(2, "0")}`;
+}
+
+/** DD.MM.YYYY in local time (Czech date format) */
+export function localDateCzStr(d: Date): string {
+  return `${String(d.getDate()).padStart(2, "0")}.${String(d.getMonth() + 1).padStart(2, "0")}.${d.getFullYear()}`;
+}
+
+/** DD.MM.YYYY HH:MM:SS in local time (Czech datetime format) */
+export function localDateTimeCzStr(d: Date): string {
+  const h = String(d.getHours()).padStart(2, "0");
+  const min = String(d.getMinutes()).padStart(2, "0");
+  const s = String(d.getSeconds()).padStart(2, "0");
+  return `${localDateCzStr(d)} ${h}:${min}:${s}`;
+}