Add sticky notes (canvas annotations) support

[vvlladd28]

This PR introduces a new sticky notes feature that allows placing free-form annotation blocks directly on the flowchart canvas. Notes can be moved, resized, selected, and deleted independently of nodes and edges.

**New public API

Models (ngx-flowchart.models.ts)**

• FcNote — new interface for a note entity (id, x, y, width, height, readonly?, open-ended extra fields)
• FcModel.notes?: FcNote[] — optional notes array on the flowchart model
• FC_NOTE_COMPONENT_CONFIG / FcNoteComponentConfig — injection token to provide a custom note renderer (mirrors the existing FC_NODE_COMPONENT_CONFIG pattern)
• UserNoteCallbacks — callback bag (noteEdit, doubleClick, mouseEnter, mouseLeave)
• UserCallbacks.noteCallbacks and UserCallbacks.noteRemoved — wired into the main callbacks object
• FcItemInfo.note — hit-test now returns the note under the pointer

Components

• FcNoteContainerComponent () — container that positions, selects, and manages drag/resize of a note; dynamically renders the configured note component type
• FcNoteComponent — abstract base directive for custom note renderers (mirrors FcNodeComponent)
• DefaultFcNoteComponent — built-in yellow sticky-note renderer used when no custom config is provided

Service

• FcNoteDraggingService — mouse-based drag and resize service with:
  • Pending mode (drag threshold before committing selection)
  • Magnet behaviour — dragging an unselected note automatically picks up nodes and nested notes whose centers fall within the note's bounds
  • Group drag — when a note is already selected, only the current selection moves
  • Three resize handles: SE corner, S edge, E edge
  • NoteDragMode enum exported for custom renderers

Model service (model.service.ts)

• NotesModel — manages note selection, deletion, and group selection (integrated into selectAll, deleteSelected, selectAllInRect)
• getNoteAtPoint, getNodesInNoteBounds, getNotesInNoteBounds — new spatial query helpers
• noteRemovedCallback — optional constructor parameter, fires after a note is deleted

Backwards compatibility

All changes are additive. FcModel.notes is optional — existing models without notes continue to work unchanged. The noteRemovedCallback constructor parameter is also optional.