fix: add imageBaseDir state for linked doc base resolution

The handleOpenLinkedDoc callback references imageBaseDir as a
fallback for first-hop relative path resolution, but the state
variable and its wiring from the /api/plan response were missing
on this branch (they exist on main via #260). Add them here so
the branch works standalone and merges cleanly.

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

Author: backnotprop

packages/editor/App.tsx

@@ -393,6 +393,7 @@ const App: React.FC = () => {
   const [origin, setOrigin] = useState<'claude-code' | 'opencode' | 'pi' | null>(null);
   const [globalAttachments, setGlobalAttachments] = useState<ImageAttachment[]>([]);
   const [annotateMode, setAnnotateMode] = useState(false);
+  const [imageBaseDir, setImageBaseDir] = useState<string | undefined>(undefined);
   const [isLoading, setIsLoading] = useState(true);
   const [isSubmitting, setIsSubmitting] = useState(false);
   const [submitted, setSubmitted] = useState<'approved' | 'denied' | null>(null);
@@ -634,12 +635,15 @@ const App: React.FC = () => {
         if (!res.ok) throw new Error('Not in API mode');
         return res.json();
       })
-      .then((data: { plan: string; origin?: 'claude-code' | 'opencode' | 'pi'; mode?: 'annotate'; sharingEnabled?: boolean; shareBaseUrl?: string; pasteApiUrl?: string; repoInfo?: { display: string; branch?: string }; previousPlan?: string | null; versionInfo?: { version: number; totalVersions: number; project: string } }) => {
+      .then((data: { plan: string; origin?: 'claude-code' | 'opencode' | 'pi'; mode?: 'annotate'; filePath?: string; sharingEnabled?: boolean; shareBaseUrl?: string; pasteApiUrl?: string; repoInfo?: { display: string; branch?: string }; previousPlan?: string | null; versionInfo?: { version: number; totalVersions: number; project: string } }) => {
         if (data.plan) setMarkdown(data.plan);
         setIsApiMode(true);
         if (data.mode === 'annotate') {
           setAnnotateMode(true);
         }
+        if (data.filePath) {
+          setImageBaseDir(data.filePath.replace(/\/[^/]+$/, ''));
+        }
         if (data.sharingEnabled !== undefined) {
           setSharingEnabled(data.sharingEnabled);
         }