fix(command): keyword/radius input for ribbon-started modify commands (#90)

Starting FILLET/CHAMFER from the ribbon left the command-line field
unfocused, so its Enter arrived as CommandFinalize and discarded a
typed option keyword ("R"/"D"), cancelling the command. Several
related dynamic-input issues surfaced from the same flow:

- CommandFinalize now forwards a non-empty command-line buffer through
  CommandSubmit, so a typed keyword is handled the same whether the
  command was started from the ribbon or the command line.
- sync_dyn_fields shows no coordinate box during an object-pick step
  (needs_entity_pick / needs_structure_point_pick) — there is no point
  to enter there, so typed keywords reach the command line cleanly.
- apply_cmd_result rebuilds the dynamic-input fields on NeedPoint, so a
  step change (object pick -> radius entry) shows the matching box
  immediately and typed digits land in it instead of the command line.
- Object-pick steps now draw a prompt-only hint pill at the cursor
  (no input box).
- FILLET/CHAMFER remember the first pick across a mid-selection radius
  or distance entry, resuming the second pick instead of restarting
  selection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -691,8 +691,18 @@ impl OpenCADStudio {
         // between boxes; typing locks a box to a fixed value while the
         // rest keep tracking the cursor. The field set is maintained in
         // `tab.dyn_fields` by `sync_dyn_fields`.
+        // A pick step (object selection) has no input box, but still shows
+        // its prompt ("Select first object …") near the cursor as a hint.
+        let dyn_picks_object = tab
+            .active_cmd
+            .as_ref()
+            .map(|c| c.needs_entity_pick() || c.needs_structure_point_pick())
+            .unwrap_or(false);
         let dyn_input_overlay: Option<Element<'_, Message>> =
-            if self.dyn_input && tab.active_cmd.is_some() && !tab.dyn_fields.is_empty() {
+            if self.dyn_input
+                && tab.active_cmd.is_some()
+                && (!tab.dyn_fields.is_empty() || dyn_picks_object)
+            {
                 let w = tab.last_cursor_world;
                 let base = self.last_point;
                 // A command may drive a typed scalar by mouse (e.g. a