fix: address PR review feedback for validation functions

1. Fix Friday logic edge case comment
   - Clarified get_default_backtest_dates() docstring
   - Removed confusing 'at least a week old' comment
   - Explicitly documented Friday behavior

2. Reorder validation checks in validate_date_range_for_symbol()
   - Now checks start > end first (most fundamental)
   - Then checks end > today (future dates)
   - Finally checks symbol-specific bounds
   - Users get most actionable error first

3. Add server-side numeric bounds validation
   - New validate_numeric_inputs() function
   - Validates units > 0, loan >= 0, 0 < LTV < 1
   - Called in run_backtest() before service call

4. Add boundary tests
   - Test start_date exactly at SYMBOL_MIN_DATES boundary
   - Test same-day date range (start == end)
   - Test end_date exactly today
   - Test end_date tomorrow (future)
   - Test validation order returns most actionable error
   - Test near-zero and large values for units calculation
   - Test LTV at boundaries (0, 1, 0.01, 0.99)

5. Add tests for validate_numeric_inputs
   - Valid inputs, zero/negative values
   - LTV boundary conditions

app/pages/backtests.py

@@ -49,13 +49,18 @@ SYMBOL_MIN_DATES = {
 }
 
 def get_default_backtest_dates() -> tuple[date, date]:
-    """Get default backtest date range (last 2 years excluding current week)."""
+    """Get default backtest date range (~2 years ending on most recent Friday or earlier).
+    
+    Returns dates (start, end) where:
+    - end is the most recent Friday (including today if today is Friday)
+    - start is ~730 days before end
+    """
     today = date.today()
-    # Find the most recent Friday that's at least a week old
+    # Find days since most recent Friday
     days_since_friday = (today.weekday() - 4) % 7
-    if days_since_friday == 0 and today.weekday() != 4:
-        # Not Friday yet, go back to previous Friday
-        days_since_friday = 7
+    # If today is Friday (weekday 4), days_since_friday is 0, so end = today
+    # If today is Saturday (weekday 5), days_since_friday is 1, so end = yesterday (Friday)
+    # etc.
     end = today - timedelta(days=days_since_friday)
     start = end - timedelta(days=730)  # ~2 years
     return start, end
@@ -69,14 +74,37 @@ def validate_date_range_for_symbol(start_date: date, end_date: date, symbol: str
     """Validate date range is within available data for symbol.
 
     Returns error message if invalid, None if valid.
+    
+    Validation order:
+    1. Logical order (start <= end)
+    2. End not in future
+    3. Symbol-specific data availability
     """
+    if start_date > end_date:
+        return "Start date must be before or equal to end date."
+    if end_date > date.today():
+        return "End date cannot be in the future."
     min_date = SYMBOL_MIN_DATES.get(symbol)
     if min_date and start_date < min_date:
         return f"Start date must be on or after {min_date.strftime('%Y-%m-%d')} for {symbol} (data availability)."
-    if end_date > date.today():
-        return "End date cannot be in the future."
-    if start_date > end_date:
-        return "Start date must be before or equal to end date."
+    return None
+
+
+def validate_numeric_inputs(
+    units: float,
+    loan_amount: float,
+    margin_call_ltv: float,
+) -> str | None:
+    """Validate numeric inputs for backtest scenario.
+
+    Returns error message if invalid, None if valid.
+    """
+    if units <= 0:
+        return "Underlying units must be positive."
+    if loan_amount < 0:
+        return "Loan amount cannot be negative."
+    if not (0 < margin_call_ltv < 1):
+        return "Margin call LTV must be between 0 and 1 (exclusive)."
     return None
 
 
@@ -781,6 +809,16 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     render_result_state("Scenario validation failed", date_range_error, tone="warning")
                     return
 
+                # Validate numeric inputs
+                units = float(units_input.value or 0.0)
+                loan = float(loan_input.value or 0.0)
+                ltv = float(ltv_input.value or 0.0)
+                numeric_error = validate_numeric_inputs(units, loan, ltv)
+                if numeric_error:
+                    validation_label.set_text(numeric_error)
+                    render_result_state("Input validation failed", numeric_error, tone="warning")
+                    return
+
                 # Save settings before running
                 save_backtest_settings()
 
@@ -789,9 +827,9 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     start_date=start_date,
                     end_date=end_date,
                     template_slug=str(template_select.value or ""),
-                    underlying_units=float(units_input.value or 0.0),
-                    loan_amount=float(loan_input.value or 0.0),
-                    margin_call_ltv=float(ltv_input.value or 0.0),
+                    underlying_units=units,
+                    loan_amount=loan,
+                    margin_call_ltv=ltv,
                 )
                 # Update cost in saved settings after successful run
                 if str(data_source_select.value) == "databento":