Fix mission file save error caused by fs module usage

Fixes the "fs is not defined" ReferenceError when saving mission files
from the Mission Control tab.

**Problem:**
- Legacy code was using Node.js fs.writeFile() directly from renderer
- fs module is not accessible in Electron's renderer process
- Users got error: "ReferenceError: fs is not defined" at line 4050

**Solution:**
- Replace fs.writeFile() with window.electronAPI.writeFile()
- Use Promise-based API (.then) instead of callback pattern
- Move success messages inside .then() block to ensure they execute
  after file write completes

**Testing:**
- Configurator builds without errors
- Mission file save dialog opens correctly
- Files save successfully to selected location
- No console errors

Follows the same pattern used in cli.js and other tabs.

Author: sensei-hacker

tabs/mission_control.js

@@ -4057,15 +4057,15 @@ function iconKey(filename) {
         var builder = new xml2js.Builder({ 'rootName': 'mission', 'renderOpts': { 'pretty': true, 'indent': '\t', 'newline': '\n' } });
         var xml = builder.buildObject(data);
         xml = xml.replace(/missionitem mission/g, 'meta mission');
-        fs.writeFile(filename, xml, (err) => {
+        window.electronAPI.writeFile(filename, xml).then(err => {
             if (err) {
                 GUI.log(i18n.getMessage('ErrorWritingFile'));
                 return console.error(err);
             }
+            let sFilename = String(filename.split('\\').pop().split('/').pop());
+            GUI.log(sFilename + i18n.getMessage('savedSuccessfully'));
+            updateFilename(sFilename);
         });
-        let sFilename = String(filename.split('\\').pop().split('/').pop());
-        GUI.log(sFilename + i18n.getMessage('savedSuccessfully'));
-        updateFilename(sFilename);
     }
 
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