Add sorting, user assignment, and UI improvements

Features:
- Add work item sorting by ID, Type, State (keys 1, 2, 3)
- Add user assignment modal with team member filtering (key a)
- Add parent work item title display in details panel
- Preserve sort order and selection on panel reload

UI improvements:
- Remove zebra striping from work items list
- Remove priority column from list and details
- Align metadata fields in details panel
- Add markdown rendering for descriptions (using glamour)
- Add state colors: To Do (orange), In Progress (purple), Done (green), Testing (yellow)

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

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

internal/models/team.go

@@ -0,0 +1,8 @@
+package models
+
+// TeamMember represents a team member
+type TeamMember struct {
+	ID          string `json:"id"`
+	DisplayName string `json:"displayName"`
+	UniqueName  string `json:"uniqueName"`
+}