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/main.js

@@ -1457,6 +1457,21 @@ function createMenu() {
           label: 'Document Compare',
           click: () => mainWindow.webContents.send('show-document-compare'),
         },
+        {
+          type: 'separator',
+        },
+        {
+          label: 'Image Tools...',
+          click: () => mainWindow.webContents.send('show-image-converter'),
+        },
+        {
+          label: 'Audio Tools...',
+          click: () => mainWindow.webContents.send('show-audio-converter'),
+        },
+        {
+          label: 'Video Tools...',
+          click: () => mainWindow.webContents.send('show-video-converter'),
+        },
       ],
     },
     {