Fix read-only banner showing for editor members

The view-only banner checked canEdit (hardcoded to isOwner) instead
of canEditTasks (which includes create_modify members). Editors saw
the banner incorrectly. Removed stale canEdit variable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/projects/ProjectDetail.tsx

@@ -146,7 +146,6 @@ export default function ProjectDetail() {
   const isShared = project ? project.user_id !== currentUserId : false;
   // For now, if they can see the project but don't own it, check if they can edit
   // The backend enforces actual permissions — this is just UI gating
-  const canEdit = isOwner; // Members with create_modify can also edit tasks (handled per-task)
   const canManageProject = isOwner;
 
   // Delta polling for real-time sync — only for shared projects (P-04)
@@ -570,7 +569,7 @@ export default function ProjectDetail() {
         </div>
 
         {/* Read-only banner for viewers */}
-        {isShared && !canEdit && (
+        {isShared && !canEditTasks && (
           <div className="mx-4 md:mx-6 mb-3 px-3 py-2 rounded-md bg-secondary/50 border border-border flex items-center gap-2">
             <Eye className="h-3.5 w-3.5 text-muted-foreground shrink-0" />
             <span className="text-xs text-muted-foreground">You have view-only access to this project</span>