fix(ai): normalize gradient stop offsets to prevent NaN display

AI sometimes omits the offset field on gradient stops, causing
Math.round(undefined * 100) = NaN in the UI. Fix at two layers:
- normalize-pen-file.ts: auto-distribute missing offsets evenly across
  [0,1] and convert percentage-format offsets (>1) to 0-1 range
- fill-section.tsx: defensive guard in the stop percentage display

src/components/panels/fill-section.tsx

@@ -221,7 +221,7 @@ export default function FillSection({
                   onChange={(c) => handleStopColorChange(i, c)}
                 />
                 <NumberInput
-                  value={Math.round(stop.offset * 100)}
+                  value={Math.round((Number.isFinite(stop.offset) ? stop.offset : i / Math.max(currentStops.length - 1, 1)) * 100)}
                   onChange={(v) => handleStopOffsetChange(i, v)}
                   min={0}
                   max={100}

src/utils/normalize-pen-file.ts

@@ -156,19 +156,31 @@ function normalizeSingleFill(
 function normalizeGradientStops(
   raw: unknown[] | undefined,
 ): GradientStop[] {
-  if (!Array.isArray(raw)) return []
-  return raw.map((s: unknown) => {
+  if (!Array.isArray(raw) || raw.length === 0) return []
+
+  // First pass: parse offsets, collecting which ones are explicitly set
+  const parsed = raw.map((s: unknown) => {
     const stop = s as Record<string, unknown>
+    const rawOffset =
+      typeof stop.offset === 'number' && Number.isFinite(stop.offset)
+        ? stop.offset
+        : typeof stop.position === 'number' && Number.isFinite(stop.position)
+          ? stop.position
+          : null
+    // Normalize percentage-format offsets (AI sometimes outputs 0-100 instead of 0-1)
+    const offset = rawOffset !== null && rawOffset > 1 ? rawOffset / 100 : rawOffset
     return {
-      offset:
-        typeof stop.offset === 'number' && Number.isFinite(stop.offset)
-          ? stop.offset
-          : typeof stop.position === 'number' && Number.isFinite(stop.position)
-            ? stop.position
-            : 0,
+      offset,
       color: typeof stop.color === 'string' ? stop.color : '#000000',
     }
   })
+
+  // Second pass: auto-distribute any stops that are missing an offset
+  const n = parsed.length
+  return parsed.map((s, i) => ({
+    color: s.color,
+    offset: s.offset !== null ? Math.max(0, Math.min(1, s.offset!)) : i / Math.max(n - 1, 1),
+  }))
 }
 
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