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/command/mod.rs

@@ -241,6 +241,19 @@ pub trait CadCommand: Send {
         false
     }
 
+    /// Returns `true` when the active text prompt expects free-form prose
+    /// that can legitimately contain whitespace (the body of a TEXT /
+    /// MTEXT / DDEDIT entity, an attribute default value, etc.). For
+    /// these prompts the command-line input must let `Space` be typed as
+    /// a literal character; for every other prompt `Space` submits the
+    /// input the same way `Enter` does.
+    ///
+    /// Default `false` — single-token prompts (option letters, numeric
+    /// radius, block name) do not embed spaces.
+    fn wants_text_with_spaces(&self) -> bool {
+        false
+    }
+
     /// Called when the user submits text via the command line while `wants_text_input` is true.
     fn on_text_input(&mut self, _text: &str) -> Option<CmdResult> {
         None