fix(forms): handle nullable fields in deepMergeWithDefaults

When default value is null but source has a value (e.g., description
field), the merge was discarding the source value because typeof null
!== typeof string. Now properly accepts source values for nullable fields.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/lib/forms/utils/mergeWithDefaults.ts

@@ -111,6 +111,10 @@ export function deepMergeWithDefaults<T extends Record<string, unknown>>(
     if (typeof sourceValue === typeof defaultValue) {
       result[key] = sourceValue as T[keyof T];
     }
+    // Special case: default is null but source has a value (nullable fields)
+    else if (defaultValue === null && sourceValue !== null) {
+      result[key] = sourceValue as T[keyof T];
+    }
     // Special case: allow null for nullable fields
     else if (sourceValue === null && defaultValue === null) {
       result[key] = null as T[keyof T];