fix(electrobun): no double dialog on cancel, browser closes on select + max-height

- pickDirectory: removed Electrobun fallback on zenity cancel (was causing
  second dialog to appear). Cancel = return null, no fallback.
- PathBrowser: handleBrowserSelect closes browser after selection
- PathBrowser container: max-height 16rem with overflow scroll + border

ui-electrobun/src/mainview/ProjectWizard.svelte

@@ -210,6 +210,7 @@
 
   function handleBrowserSelect(path: string) {
     localPath = path;
+    showBrowser = false;
     validatePath(path);
   }
 
@@ -304,7 +305,7 @@
               </span>
             {/if}
           </div>
-          <div style:display={showBrowser ? 'block' : 'none'}>
+          <div style:display={showBrowser ? 'block' : 'none'} style="max-height: 16rem; overflow-y: auto; border: 1px solid var(--ctp-surface1); border-radius: 0.375rem; margin-top: 0.5rem;">
             <PathBrowser onSelect={handleBrowserSelect} onClose={() => showBrowser = false} />
           </div>
           {#if pathValid === 'valid' && isGitRepo}