feat: Add accounting period selector for all views

- Add PeriodSelectorMini component for global period selection
- Add accountingPeriod store for shared period state
- Add calendar service/router/model for available periods API
- Update Dashboard, Invoices, Trial Balance, Bank/Cash Register views
  to respect selected period
- Fix Trial Balance navigation sync bug (period now syncs on mount)
- Update backend services to accept luna/an parameters

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

reports-app/backend/app/models/calendar.py

@@ -0,0 +1,19 @@
+"""
+Calendar period models for accounting period selector
+"""
+from pydantic import BaseModel
+from typing import List, Optional
+
+
+class CalendarPeriod(BaseModel):
+    """Model for an accounting period"""
+    an: int           # Year
+    luna: int         # Month (1-12)
+    display_name: str # Format: "Decembrie 2025"
+
+
+class CalendarPeriodsResponse(BaseModel):
+    """Response model for calendar periods list"""
+    periods: List[CalendarPeriod]
+    current_period: Optional[CalendarPeriod] = None  # Most recent period
+    total_count: int