feat(commandline): always-on keyboard capture + space submits like enter

Two behaviour changes that bring the command line in line with
established CAD workflow:

1. **Always-on keyboard capture.** The input was blurred whenever no
   command was running, so typed letters went nowhere until the user
   clicked into the field. Replace the blur with a re-focus on every
   command lifecycle transition (cmd_result.rs) and on viewport / tab
   / ribbon clicks (update.rs), focus the input at startup, and add
   a global key subscription that catches any unhandled character key
   or backspace and routes it into the buffer via two new messages
   (CommandAppendChar / CommandBackspace) — so the command line keeps
   accumulating keystrokes even if focus parks somewhere not
   focusable. Removes the unused blur_cmd_input helper.

2. **Space submits.** Typing a space at the end of the input now
   submits the buffer the same way Enter does. Only exception: an
   active command that's currently collecting free-form prose
   (TEXT / MTEXT / DDEDIT body, ATTDEF prompt / default value,
   INSERT attribute value, ATTEDIT cell). Those advertise the new
   `CadCommand::wants_text_with_spaces()` flag so the Space character
   reaches the buffer as a literal.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/app/cmd_result.rs

@@ -1539,13 +1539,14 @@ impl OpenCADStudio {
                 self.restore_pre_cmd_tangent();
             }
         }
-        // Focus the command-line input while a command is active; blur it when the command ends.
-        if self.tabs[i].active_cmd.is_some() {
-            self.focus_cmd_input()
-        } else {
+        // Keep the command-line input focused at all times — every typed
+        // character is meant to route there (the command processor reads
+        // its keystroke stream from this widget). When no command is
+        // running the ribbon tool button still has to visually deactivate.
+        if self.tabs[i].active_cmd.is_none() {
             self.ribbon.deactivate_tool();
-            self.blur_cmd_input()
         }
+        self.focus_cmd_input()
     }
 
     /// Restore the tangent snap state that was in effect before the command started.