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/cmd_result.rs

@@ -42,6 +42,12 @@ impl OpenCADStudio {
                 if let Some(p) = prompt {
                     self.command_line.push_info(&p);
                 }
+                // The command may have advanced to a step with a different
+                // dynamic-input shape (e.g. FILLET object-pick → radius entry).
+                // Rebuild the fields now so the matching box appears immediately
+                // and typed digits land in it rather than the command line,
+                // instead of waiting for the next cursor move to resync.
+                self.sync_dyn_fields();
             }
             CmdResult::Preview(wire) => {
                 self.tabs[i].scene.set_preview_wires(vec![wire]);