feat: PRINT command — send layout to system printer via lp/lpr

The PRINT command renders the current layout to a temporary PDF (reusing
the existing PDF export pipeline) and dispatches it to the OS default
printer:
  - Linux/macOS: `lp` (CUPS) with `lpr` as fallback.
  - Windows: ShellExecute "print" verb (compiled-in, unused on Linux).

PLOT and EXPORT still open the save-file dialog as before.
Status messages are shown in the command line during the async job.

Closes ROADMAP 1.8.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/commands.rs

@@ -3794,9 +3794,13 @@ impl H7CAD {
             }
 
             // ── Plot / Page Setup ──────────────────────────────────────────
-            "PRINT"|"PLOT"|"EXPORT" => {
+            "PLOT"|"EXPORT" => {
                 return Task::done(Message::PlotExport);
             }
+            // PRINT — send current layout to the system default printer.
+            "PRINT" => {
+                return Task::done(Message::PrintToPrinter);
+            }
             // PLOTSTYLE — load or clear CTB/STB plot style table
             cmd if cmd == "PLOTSTYLE" || cmd.starts_with("PLOTSTYLE ") => {
                 let sub = cmd.split_once(' ')