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/components/layout/AppSidebar.vue

@@ -182,6 +182,7 @@ import {
   Package,
   ListTodo,
   ShieldCheck,
+  GanttChartSquare,
 } from 'lucide-vue-next'
 
 import { useAuthStore } from '@/stores/auth'
@@ -221,6 +222,7 @@ const projectTabs = computed(() => {
     { id: 'shots', label: 'Shots', icon: Camera, route: `/projects/${id}/shots` },
     { id: 'assets', label: 'Assets', icon: Package, route: `/projects/${id}/assets` },
     { id: 'tasks', label: 'Tasks', icon: ListTodo, route: `/projects/${id}/tasks` },
+    { id: 'schedule', label: 'Schedule', icon: GanttChartSquare, route: `/projects/${id}/schedule` },
     { id: 'settings', label: 'Settings', icon: Settings, route: `/projects/${id}/settings` },
   ]
 })
@@ -233,6 +235,7 @@ const activeProjectTab = computed(() => {
   if (path.startsWith(`/projects/${id}/shots`)) return 'shots'
   if (path.startsWith(`/projects/${id}/assets`)) return 'assets'
   if (path.startsWith(`/projects/${id}/tasks`)) return 'tasks'
+  if (path.startsWith(`/projects/${id}/schedule`)) return 'schedule'
   if (path.startsWith(`/projects/${id}/settings`)) return 'settings'
   return null
 })