Tree rows get the same right-click actions as canvas boxes

Right-clicking a component in the navigation tree offered Delete Component
alone, while the same component on the wiresheet also offered Link Mark and
Link From / Link To. Which list a component was picked from should not change
what can be done with it, so both popups are now built by
fill_component_popup.

Its entries close over the component instead of reading a selection back,
which is what lets each popup act on the row or box that was actually clicked
-- the canvas commands went through app_state.selected_canvas_tag, which a
tree row never sets. do_link_mark, do_link_from_mark, do_link_to_mark and
delete_tree_selected_component fall out with it, and Link Mark now says in the
status bar what it marked, since nothing else on screen shows that.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -128,6 +128,14 @@ element has left the document, keeping the position on screen. Walking the histo
 the tree selection, which is what paints the sheet, and is suppressed from recording so a
 walk cannot rewrite the trail it is following.
 
+### The component popup
+
+Right-clicking a component offers the same actions from the wiresheet and from the
+navigation tree -- Delete Component, Link Mark, and Link From / Link To naming the marked
+component once one is marked. `fill_component_popup` builds both, and its entries close
+over the component rather than reading a selection back, which is what lets each popup act
+on the row or box that was actually clicked. The tree's root row keeps its own popup.
+
 ### Following a link to its other sheet
 
 Right-clicking a wire or a knob offers **Go to Link Wire Sheet "<name>"** alongside Delete