Add editable status column and submission markers to the Gantt chart

Adds a second frozen "Task Status" column (EditableTaskStatus) next to
the task name, and small white dot markers on each bar showing
submission dates - backed by a new lightweight GET /tasks/submission-dates
endpoint (task_id + submitted_at only, to avoid an N+1 fetch across
potentially hundreds of scheduled tasks).

Restructures the chart's scroll handling from a single scrolling
container with sticky-positioned columns to two independently-scrolled
panes (frozen name/status columns, and the timeline), synced on
vertical scroll. This fixes several issues that came with the sticky
approach - transparency bleed-through on hover and on group-header
rows, row heights not matching between columns, and the horizontal
scrollbar spanning the frozen columns instead of just the timeline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/services/task.ts

@@ -109,6 +109,11 @@ export interface Submission {
   stream_url?: string
 }
 
+export interface SubmissionDateInfo {
+  task_id: number
+  submitted_at: string
+}
+
 export interface TaskFilters {
   projectId?: number
   shotId?: number
@@ -249,6 +254,11 @@ class TaskService {
     return response.data
   }
 
+  async getSubmissionDates(projectId: number): Promise<SubmissionDateInfo[]> {
+    const response = await apiClient.get(`/tasks/submission-dates?project_id=${projectId}`)
+    return response.data
+  }
+
   async submitWork(taskId: number, file: File, notes?: string): Promise<Submission> {
     const formData = new FormData()
     formData.append('file', file)