Route downloads into Documents\Video / Documents\Audio and simplify the home page

- Create Documents\Video and Documents\Audio on startup (ensureMediaDirs), and
  route each download into them by kind when no explicit output folder is set.
  An empty outputDir now means "sort by type"; a chosen folder still overrides
  for both kinds.
- Settings/Onboarding "Download folder" gains a placeholder + hint describing
  the blank = route-by-type behaviour.
- Trim the home download bar to the essentials: URL box, Search and Paste
  buttons, format/quality, and the Download button. Removed the private-mode
  toggle, per-download Options panel, custom-command panel, command preview, and
  the saving-to-folder line.
- Bump version to 0.3.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "aerofetch",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "description": "A yt-dlp frontend for Windows",
   "main": "./out/main/index.js",
   "author": "AeroFetch",