feat: implement custom fields, gantt view, calendar view, and file encryption

- Custom Fields (FEAT-001):
  - CustomField and TaskCustomValue models with formula support
  - CRUD API for custom field management
  - Formula engine for calculated fields
  - Frontend: CustomFieldEditor, CustomFieldInput, ProjectSettings page
  - Task list API now includes custom_values
  - KanbanBoard displays custom field values

- Gantt View (FEAT-003):
  - TaskDependency model with FS/SS/FF/SF dependency types
  - Dependency CRUD API with cycle detection
  - start_date field added to tasks
  - GanttChart component with Frappe Gantt integration
  - Dependency type selector in UI

- Calendar View (FEAT-004):
  - CalendarView component with FullCalendar integration
  - Date range filtering API for tasks
  - Drag-and-drop date updates
  - View mode switching in Tasks page

- File Encryption (FEAT-010):
  - AES-256-GCM encryption service
  - EncryptionKey model with key rotation support
  - Admin API for key management
  - Encrypted upload/download for confidential projects

- Migrations: 011 (custom fields), 012 (encryption keys), 013 (task dependencies)
- Updated issues.md with completion status

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/archive/2026-01-05-add-calendar-view/tasks.md

@@ -0,0 +1,80 @@
+# Tasks: Add Calendar View
+
+## Backend Tasks
+
+### 1. Add date range filter to Tasks API
+- [x] Modify GET /api/projects/{project_id}/tasks to accept `due_after` and `due_before` query params
+- [x] Add filter logic to query
+- [x] Add API tests
+- **驗證**: 可依日期範圍篩選任務 ✅
+
+## Frontend Tasks
+
+### 2. Install and configure FullCalendar
+- [x] Install @fullcalendar/react and related packages
+- [x] Install @fullcalendar/daygrid (month view)
+- [x] Install @fullcalendar/timegrid (week/day view)
+- [x] Install @fullcalendar/interaction (drag & drop)
+- **驗證**: 套件安裝成功 ✅
+
+### 3. Create CalendarView component
+- [x] Create `frontend/src/components/CalendarView.tsx`
+- [x] Configure calendar with month/week/day views
+- [x] Style calendar to match application theme
+- [x] Add view toggle buttons
+- **驗證**: 行事曆正確渲染 ✅
+
+### 4. Load and display tasks on calendar
+- [x] Fetch tasks from API with date range
+- [x] Transform tasks to FullCalendar event format
+- [x] Display task title, status color, priority icon
+- [x] Handle overdue tasks (special styling)
+- **驗證**: 任務正確顯示在對應日期 ✅
+
+### 5. Implement task click to open detail modal
+- [x] Handle event click in calendar
+- [x] Open TaskDetailModal with selected task
+- [x] Refresh calendar on task update
+- **驗證**: 點擊任務開啟詳情 ✅
+
+### 6. Implement drag-to-change date
+- [x] Enable drag & drop in FullCalendar
+- [x] Handle event drop to get new date
+- [x] Call PUT /api/tasks/{id} with new due_date
+- [x] Show optimistic update, rollback on error
+- **驗證**: 拖拉更新日期可正常運作 ✅
+
+### 7. Add filter controls
+- [x] Add assignee filter dropdown
+- [x] Add status filter (show/hide completed)
+- [x] Add priority filter
+- [x] Persist filter in URL or localStorage
+- **驗證**: 篩選器正確過濾任務 ✅
+
+### 8. Integrate Calendar view into Tasks page
+- [x] Add "Calendar" option to view toggle
+- [x] Store view preference in localStorage
+- [x] Handle view switching
+- **驗證**: 可在 List/Kanban/Gantt/Calendar 之間切換 ✅
+
+## Task Dependencies
+
+```
+[1] Date Range Filter API
+      ↓
+[2] FullCalendar Setup → [3] CalendarView Component
+                                    ↓
+                          [4] Task Display
+                                    ↓
+                          [5] Click → Modal
+                                    ↓
+                          [6] Drag to Change Date
+                                    ↓
+                          [7] Filters
+                                    ↓
+                          [8] View Integration
+```
+
+- Task 1 (Backend) 可平行於 Tasks 2-3 (Frontend) 開發
+- Task 4 需要 Task 1 完成（需要日期範圍 API）
+- Tasks 5-8 為循序開發