Add link management: create, select, delete with undo/redo

Implement full link workflow between component slots on the wiresheet:
- Link Mark / Link From / Link To context menu items for creating links
- Link dialog with slot type validation and source/target swap button
- Selectable link wires (click to highlight in red) with right-click delete
- Undo/redo support for both add and delete link operations
- Component deletion already cascades to remove related links

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app_state.py

@@ -36,6 +36,9 @@ current_file_path = None
 active_components_list = []   # Tracks what components are actively painted
 selected_canvas_tag = None    # Tracks the currently clicked/highlighted canvas component tag
 selected_tree_item_id = None  # Tracks the currently right-clicked Treeview item
+link_mark_element = None      # Component marked as a link endpoint via "Link Mark"
+selected_link_tag = None      # Canvas tag of the currently selected (red-highlighted) link wire
+canvas_link_map = {}          # Canvas link tag -> ET.Element (<link>)
 xml_root_element = None       # Keep a persistent root reference to redraw tree layouts cleanly
 current_sheet_parent = None   # Element whose children are painted; None means the app root