feat(media): add batch folder mode to Image/Audio/Video Tools dialog

Reviewer follow-up on Task 12: the task's own title/brief called for
batch support and no later task in the plan picks it up, so this closes
that gap. Adds a "Single File" / "Batch Folder" mode toggle to the
existing media-operations-dialog.js; batch mode swaps the per-file
input/output fields for an Input Folder + "Include subfolders" +
Output Folder trio while keeping every other parameter (width/height/
quality/angle/startTime/duration/crf/fps/format/fit) applied uniformly
to every matching file. Disabled for audio "Merge", which combines many
inputs into one output and doesn't fit a per-file batch model.

main.js: adds collectFilesByExtension() (src/main/collectFilesByExtension.js,
unit tested), a generalization of the inline collectFiles() closure inside
ipcMain.on('universal-convert-batch', ...) to match a set of extensions
instead of one format. runMediaBatchOperation() loops
ImageOperations/AudioOperations/VideoOperations.executeOperation() over
the matched files, reporting per-file progress via new
'media-batch-progress' events and a final 'media-batch-complete' event,
then shows a "Batch Conversion Complete" dialog.showMessageBox with
completed/failed counts, mirroring performBatchConversion()'s pattern.
Wired via three new ipcMain.on handlers: batch-image-operation,
batch-audio-operation, batch-video-operation.

preload.js: whitelists the three new send channels and the two new
receive channels (media-batch-progress, media-batch-complete).

src/main/collectFilesByExtension.js

@@ -0,0 +1,52 @@
+/**
+ * collectFilesByExtension
+ *
+ * Recursively (optionally) collects files under a directory whose extension matches
+ * one of a given set of extensions. Generalizes the `collectFiles()` closure defined
+ * inside `ipcMain.on('universal-convert-batch', ...)` in main.js (which matches a
+ * single `.${fromFormat}` extension) to match against an arbitrary extension list —
+ * used by the batch-image/audio/video-operation handlers, which need to match several
+ * possible input extensions per media kind (e.g. .jpg/.jpeg/.png/... for images).
+ *
+ * Pulled out as its own module (rather than an inline closure like the original) so it
+ * can be unit tested without Electron.
+ *
+ * @module collectFilesByExtension
+ */
+
+const fs = require('fs');
+const path = require('path');
+
+/**
+ * @param {string} dir - Directory to scan.
+ * @param {string[]} extensions - Extensions to match, each including the leading dot
+ *   (e.g. ['.jpg', '.png']). Matching is case-insensitive.
+ * @param {boolean} [includeSubfolders=true] - Recurse into subdirectories.
+ * @returns {string[]} Absolute paths of matching files, in directory-walk order.
+ */
+function collectFilesByExtension(dir, extensions, includeSubfolders = true) {
+  const normalizedExts = (extensions || []).map((ext) => ext.toLowerCase());
+  const results = [];
+
+  function walk(currentDir) {
+    const entries = fs.readdirSync(currentDir, { withFileTypes: true });
+    for (const entry of entries) {
+      const fullPath = path.join(currentDir, entry.name);
+      if (entry.isDirectory()) {
+        if (includeSubfolders) {
+          walk(fullPath);
+        }
+      } else if (entry.isFile()) {
+        const ext = path.extname(entry.name).toLowerCase();
+        if (normalizedExts.includes(ext)) {
+          results.push(fullPath);
+        }
+      }
+    }
+  }
+
+  walk(dir);
+  return results;
+}
+
+module.exports = { collectFilesByExtension };