feat: custom date range filter on calendar page

- Backend: /api/eco/calendar accepts date_from + date_to query params
  when period=custom; get_calendar() uses them directly without override
  Limit raised to 5000 for wide date ranges
- Frontend: "Custom" tab at end of period list; shows two date inputs
  (from/to) with Apply button; displays day count; fetches on Apply click
  (not on every keystroke to avoid hammering the API)

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

backend/services/ff_calendar.py

@@ -232,6 +232,8 @@ def sync_live() -> Dict[str, Any]:
 
 def get_calendar(
     period: str = "recent",
+    date_from: Optional[str] = None,
+    date_to: Optional[str] = None,
     currencies: Optional[List[str]] = None,
     impacts: Optional[List[str]] = None,
     limit: int = 300,
@@ -243,6 +245,7 @@ def get_calendar(
     period options:
       recent | today | tomorrow | yesterday | this_week | next_week |
       previous_week | this_month | next_month | previous_month | custom
+    When period=custom, date_from and date_to are used directly.
     """
     from services.database import get_conn
 
@@ -250,10 +253,14 @@ def get_calendar(
     monday = today - timedelta(days=today.weekday())
     sunday = monday + timedelta(days=6)
 
-    date_from: Optional[str] = None
-    date_to:   Optional[str] = None
+    # For custom period, date_from/date_to come from the caller — don't override
+    if period != "custom":
+        date_from = None
+        date_to   = None
 
-    if period == "today":
+    if period == "custom":
+        pass  # date_from / date_to already set by caller
+    elif period == "today":
         date_from = date_to = str(today)
     elif period == "tomorrow":
         t = today + timedelta(days=1)