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 <noreply@anthropic.com>

src/app/mod.rs

@@ -343,6 +343,14 @@ pub enum Message {
     CommandInput(String),
     CommandSubmit,
     Command(String),
+    /// Append one typed character to the command-line input from the
+    /// global key-press subscription. Used when the text-input widget
+    /// itself isn't focused (focus parked on viewport / button / etc.)
+    /// so typing still routes to the command line.
+    CommandAppendChar(String),
+    /// Pop the trailing character off the command-line input — backspace
+    /// counterpart to `CommandAppendChar`.
+    CommandBackspace,
     /// Recall previous command in history (↑ arrow key).
     CommandHistoryPrev,
     /// Recall next command in history (↓ arrow key).
@@ -848,7 +856,8 @@ impl OpenCADStudio {
             crate::update_check::check_for_update(),
             Message::UpdateCheckResult,
         );
-        (s, Task::batch([open_main, check_update]))
+        let focus_cmd = s.focus_cmd_input();
+        (s, Task::batch([open_main, check_update, focus_cmd]))
     }
 }