feat: add project export and import functionality

- Implemented `pcli project export` command to export project hierarchy as JSON.
- Added `pcli project import` command to import project data from JSON.
- Created user client to fetch user details for comment attribution.
- Introduced new data structures for export and import processes.
- Ensured name-based references in exports and handled conflicts during imports.
- Added versioning and progress reporting for import operations.
- Updated documentation and specifications for new features.

model/types.go

@@ -178,6 +178,77 @@ type ListSummary struct {
 	ClosedCards int   `json:"closedCards"`
 }
 
+type User struct {
+	ID       string  `json:"id"`
+	Name     string  `json:"name"`
+	Username *string `json:"username"`
+	Email    string  `json:"email"`
+}
+
+// Export types - portable structs without Planka IDs
+
+type ExportEnvelope struct {
+	Version    int           `json:"version"`
+	ExportedAt string        `json:"exportedAt"`
+	Project    ExportProject `json:"project"`
+}
+
+type ExportProject struct {
+	Name        string        `json:"name"`
+	Description *string       `json:"description,omitempty"`
+	Boards      []ExportBoard `json:"boards"`
+}
+
+type ExportBoard struct {
+	Name     string        `json:"name"`
+	Position float64       `json:"position"`
+	Lists    []ExportList  `json:"lists"`
+	Labels   []ExportLabel `json:"labels"`
+	Cards    []ExportCard  `json:"cards"`
+}
+
+type ExportList struct {
+	Name     string  `json:"name"`
+	Position float64 `json:"position"`
+	Type     string  `json:"type"`
+	Color    *string `json:"color,omitempty"`
+}
+
+type ExportLabel struct {
+	Name     *string `json:"name,omitempty"`
+	Position float64 `json:"position"`
+	Color    string  `json:"color"`
+}
+
+type ExportCard struct {
+	Name        string           `json:"name"`
+	Position    *float64         `json:"position,omitempty"`
+	ListName    string           `json:"listName"`
+	LabelNames  []string         `json:"labelNames,omitempty"`
+	Description *string          `json:"description,omitempty"`
+	DueDate     *string          `json:"dueDate,omitempty"`
+	IsClosed    bool             `json:"isClosed,omitempty"`
+	Type        string           `json:"type"`
+	TaskLists   []ExportTaskList `json:"taskLists,omitempty"`
+	Comments    []ExportComment  `json:"comments,omitempty"`
+}
+
+type ExportTaskList struct {
+	Name     string       `json:"name"`
+	Position float64      `json:"position"`
+	Tasks    []ExportTask `json:"tasks"`
+}
+
+type ExportTask struct {
+	Name        string  `json:"name"`
+	Position    float64 `json:"position"`
+	IsCompleted bool    `json:"isCompleted"`
+}
+
+type ExportComment struct {
+	Text string `json:"text"`
+}
+
 type APIError struct {
 	StatusCode int
 	Message    string