Add project Schedule page with interactive Gantt chart

Adds a Schedule tab (Kitsu-style production schedule) under each
project: tasks grouped by type with drag-to-reschedule bars, Day/Week/
Month zoom, manual date-range control, weekend shading, a frozen task
column, and a two-tier month/date axis header. Requires a new
start_date field on Task (start_date was previously missing; only
deadline existed) and shadcn DatePicker inputs replace native date
inputs on the Schedule toolbar and TaskDetailPanel's Start Date/
Deadline fields.

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

frontend/src/services/task.ts

@@ -11,6 +11,7 @@ export interface Task {
   description?: string
   task_type: string
   status: TaskStatus
+  start_date?: string
   deadline?: string
   project_id: number
   project_name?: string
@@ -33,6 +34,7 @@ export interface TaskListItem {
   name: string
   task_type: string
   status: TaskStatus
+  start_date?: string
   deadline?: string
   project_id: number
   project_name: string
@@ -115,6 +117,7 @@ export interface TaskFilters {
   status?: string
   taskType?: string
   departmentRole?: string
+  limit?: number
 }
 
 export interface BulkStatusUpdateRequest {
@@ -143,7 +146,8 @@ class TaskService {
     if (filters?.status) params.append('status', filters.status)
     if (filters?.taskType) params.append('task_type', filters.taskType)
     if (filters?.departmentRole) params.append('department_role', filters.departmentRole)
-    
+    if (filters?.limit) params.append('limit', filters.limit.toString())
+
     const response = await apiClient.get(`/tasks/?${params}`)
     return response.data
   }