feat(media): add Image/Audio/Video Tools dialogs wired to new operation backends

Adds Tools > Image/Audio/Video Tools... menu items and a single dynamic
renderer dialog (src/renderer/media-operations-dialog.js) that lets the
user pick a media-kind-scoped operation, fill in its operation-specific
fields, and invoke process-image-operation/process-audio-operation/
process-video-operation (Tasks 9-11's backends). File selection reuses
the existing <input type="file"> + file.path convention; the one folder
picker need (video frame extraction) reuses the existing generic
select-folder/folder-selected IPC channels, so no new IPC handler was
required. Also removes the three dead electronAPI.image/audio/video
convenience blocks from preload.js (stale pre-Task-9-11 channel names,
unused everywhere).

src/renderer.js

@@ -11,6 +11,7 @@ const DOMPurify = createDOMPurify(window);
 const hljs = require('highlight.js');
 const { createEditor } = require('./editor/codemirror-setup');
 const { undo, redo } = require('@codemirror/commands');
+const { showMediaOperationsDialog } = require('./renderer/media-operations-dialog');
 
 /**
  * Toggle body classes that drive the monospace font + ligatures CSS tokens.
@@ -3723,6 +3724,17 @@ ipcRenderer.on('show-pdf-editor-dialog', (event, operation, openedFilePath) => {
   currentPDFOperation = operation;
   showPDFEditorDialog(operation, openedFilePath);
 });
+
+// Show Media Operations Dialogs (Tools > Image/Audio/Video Tools...)
+ipcRenderer.on('show-image-converter', () => {
+  showMediaOperationsDialog('image');
+});
+ipcRenderer.on('show-audio-converter', () => {
+  showMediaOperationsDialog('audio');
+});
+ipcRenderer.on('show-video-converter', () => {
+  showMediaOperationsDialog('video');
+});
 function showPDFEditorDialog(operation, openedFilePath = null) {
   const title = document.getElementById('pdf-editor-title');