feat(roadmap): surface stale checkboxes + collection output-path setting

Command preview + incognito toggle were already wired end-to-end in the
DownloadBar (Show command button, Incognito checkbox) — flipped the two
stale ROADMAP.md checkboxes with notes; verified live in the preview.

Output-path setting (PINCHFLAT): new Settings.collectionOutputTemplate — a
raw yt-dlp -o template overriding the default Channel/Playlist/NNN-Title
layout for Library downloads (empty = default). collectionOutputTemplate()
takes an optional custom template (joined under outDir, left for yt-dlp to
expand); download.ts passes the setting; zod schema validates it as a safe
relative template; Settings -> Filenames field. Tests in buildArgs +
settingsSchema; live-verified the field renders.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/shared/ipc.ts

@@ -565,6 +565,14 @@ export interface Settings {
   maxConcurrent: number
   /** yt-dlp output template, e.g. '%(title)s.%(ext)s' */
   filenameTemplate: string
+  /**
+   * Raw yt-dlp `-o` template for COLLECTION downloads (a channel/playlist from
+   * the Library), overriding the default `Channel/Playlist/NNN - Title` folder
+   * layout. Empty = the built-in default. A relative template (no absolute path,
+   * no `..`); yt-dlp expands its own fields (`%(uploader)s`, `%(playlist)s`,
+   * `%(playlist_index)s`, `%(title)s`, `%(ext)s`, …). PINCHFLAT output-path setting.
+   */
+  collectionOutputTemplate: string
   /** UI color theme: explicit light/dark, or 'system' to follow the OS */
   theme: ThemeMode
   /** brand accent preset (buttons, links, selected nav item) */
@@ -659,6 +667,7 @@ export const DEFAULT_SETTINGS: Settings = {
   defaultAudioQuality: 'Best',
   maxConcurrent: 2,
   filenameTemplate: '%(title)s.%(ext)s',
+  collectionOutputTemplate: '',
   // Follow the OS theme on first launch (SR1) so a dark-mode Windows user isn't
   // greeted by a bright white window despite full system-theme support.
   theme: 'system',