feat: hide diagnostics button if plugin not installed

Author: dkaser

src/usr/local/emhttp/plugins/tailscale/Tailscale-1-Settings.page

@@ -115,6 +115,7 @@ Tag="gears"
 : <input type="submit" value="<?= _tr('Restart') ?>">
 </form>
 
+<?php if(file_exists('/usr/local/emhttp/plugins/plugin-diagnostics/download.php')) { ?>
 ### <?= _tr("settings.diagnostics") ?>
 
 <form markdown="1" method="GET" action="/plugins/plugin-diagnostics/download.php" target="_blank">
@@ -123,6 +124,8 @@ Tag="gears"
 : <input type="submit" value="<?= _tr('Download') ?> ">
 </form>
 
+<?php } ?>
+
 <div class="advanced" markdown="1">
 ### <?= _tr("settings.erase") ?>
 

src/usr/local/emhttp/plugins/tailscale/Tailscale-5-Log.page

@@ -76,5 +76,5 @@ $(function() {
 </script>
 <span><span class='lite label'>_(Log size)_:&nbsp;&nbsp;<input type='number' id='max' value='' placeholder='<?=$max?>'></span><?=$select?></span>
 <input type="button" value="_(Refresh)_" onclick="showLog(logfile)">
-<input type="button" value="<?= _tr("settings.diagnostics") ?>" onclick="window.open('/plugins/plugin-diagnostics/download.php?plugin=tailscale','_blank')" />
+<?php if(file_exists('/usr/local/emhttp/plugins/plugin-diagnostics/download.php')) { ?> <input type="button" value="<?= _tr("settings.diagnostics") ?>" onclick="window.open('/plugins/plugin-diagnostics/download.php?plugin=tailscale','_blank')" /><?php } ?>
 <pre class='u'></pre>