Separate per-kind video/audio folders, defaulting to Documents\Video and Documents\Audio

Existing installs had outputDir persisted to the Downloads folder (auto-filled by
earlier versions), so the v0.3.0 per-kind routing never triggered — downloads kept
going to Downloads. Replace the single outputDir setting with independent videoDir
and audioDir settings:

- Settings model: drop Settings.outputDir; add videoDir + audioDir (both blank by
  default). A blank value routes that kind into Documents\Video / Documents\Audio;
  a chosen folder overrides it. The stale outputDir key in old settings files is
  simply ignored, so existing users now get the Documents defaults.
- buildCommand routes by kind: per-download override → per-kind folder → default.
- The renderer no longer sends a global outputDir, so main always routes by kind.
- Settings: the single "Download folder" field becomes separate "Video folder" and
  "Audio folder" pickers, each with a Browse + Reset and a Documents\… placeholder.
  Store gains chooseDir(target) / clearDir(target).
- Onboarding: replace the folder picker with a short note about the two default
  folders (changeable in Settings).
- Bump version to 0.3.1.

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

src/shared/ipc.ts

@@ -340,8 +340,10 @@ export type DownloadEvent =
 
 /** Persisted user settings (electron-store). */
 export interface Settings {
-  /** absolute output directory (empty string resolves to the OS Downloads folder) */
-  outputDir: string
+  /** where video downloads are saved; empty string = the default Documents\Video folder */
+  videoDir: string
+  /** where audio downloads are saved; empty string = the default Documents\Audio folder */
+  audioDir: string
   defaultKind: MediaKind
   defaultVideoQuality: string
   defaultAudioQuality: string