fix(web): seven web UI bugs, and remove dead plugins_manager.js helpers (#647)

- Operation History: the plugin filter lists the installed plugin ids
  instead of one option, "plugins" (Object.keys of {plugins: [...]}).
- Ctrl/Cmd+S submits the active tab's first visible form with
  requestSubmit() (validation and onsubmit guards run) instead of a bare
  Event on the first form in the document; skipped inside a modal dialog
  and on tabs without a form.
- Overview "Check Updates" confirms like "Update Code", takes its button
  explicitly (no implicit global event) and shows the server's message.
  Both, and the Tools tab git pull, raise the restart-pending banner on
  restart_required.
- Tools: toolsAction and diagnostics show the server's error message;
  only a non-JSON body falls back to HTTP <status>.
- Installed list after uninstall: PluginAPI writes clear the throttler's
  GET cache, a forced loadInstalledPlugins clears it too, and the
  post-uninstall reload goes through refreshInstalledPlugins().
- Plugin widgets load from /static/plugin-widgets/ only (the other two
  paths have no route).
- Raw JSON editor escapes the parse error; slider escapes value/min/max/step.
- Removed the unreferenced array-of-objects and key-value helpers from
  plugins_manager.js, the textarea auto-resize and Ctrl+R handlers in
  app.js, and a redundant ?v= on the plugins_manager.js script tag.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/static/v3/js/widgets/slider.js

@@ -85,7 +85,7 @@
                 html += `
                     <div class="flex justify-center mb-2">
                         <span id="${fieldId}_value" class="text-lg font-semibold text-gray-700">
-                            ${escapeHtml(prefix)}${currentValue}${escapeHtml(suffix)}
+                            ${escapeHtml(prefix)}${escapeHtml(currentValue)}${escapeHtml(suffix)}
                         </span>
                     </div>
                 `;
@@ -96,10 +96,10 @@
                 <input type="range"
                        id="${fieldId}_input"
                        name="${escapeHtml(options.name || fieldId)}"
-                       value="${currentValue}"
-                       min="${min}"
-                       max="${max}"
-                       step="${step}"
+                       value="${escapeAttr(currentValue)}"
+                       min="${escapeAttr(min)}"
+                       max="${escapeAttr(max)}"
+                       step="${escapeAttr(step)}"
                        ${disabled ? 'disabled' : ''}
                        oninput="window.LEDMatrixWidgets.getHandlers('slider').onInput('${fieldId}')"
                        onchange="window.LEDMatrixWidgets.getHandlers('slider').onChange('${fieldId}')"
@@ -110,8 +110,8 @@
             if (showMinMax) {
                 html += `
                     <div class="flex justify-between mt-1">
-                        <span class="text-xs text-gray-400">${escapeHtml(prefix)}${min}${escapeHtml(suffix)}</span>
-                        <span class="text-xs text-gray-400">${escapeHtml(prefix)}${max}${escapeHtml(suffix)}</span>
+                        <span class="text-xs text-gray-400">${escapeHtml(prefix)}${escapeHtml(min)}${escapeHtml(suffix)}</span>
+                        <span class="text-xs text-gray-400">${escapeHtml(prefix)}${escapeHtml(max)}${escapeHtml(suffix)}</span>
                     </div>
                 `;
             }