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>

test/buildArgs.test.ts

@@ -735,6 +735,27 @@ describe('collectionOutputTemplate', () => {
     )
     expect(t.replace(/\\/g, '/')).toBe('C:/out/A B/Untitled/001 - %(title)s.%(ext)s')
   })
+
+  it('a custom template overrides the built-in layout and is left for yt-dlp to expand', () => {
+    const t = collectionOutputTemplate(
+      'C:/out',
+      ctx,
+      '%(title)s.%(ext)s',
+      '%(uploader)s/%(playlist)s/%(playlist_index)s - %(title)s.%(ext)s'
+    )
+    expect(t.replace(/\\/g, '/')).toBe(
+      'C:/out/%(uploader)s/%(playlist)s/%(playlist_index)s - %(title)s.%(ext)s'
+    )
+  })
+
+  it('a blank/whitespace custom template falls back to the built-in layout', () => {
+    expect(collectionOutputTemplate('C:/o', ctx, 'f', '').replace(/\\/g, '/')).toBe(
+      'C:/o/DevChannel/Full Course/003 - f'
+    )
+    expect(collectionOutputTemplate('C:/o', ctx, 'f', '   ').replace(/\\/g, '/')).toBe(
+      'C:/o/DevChannel/Full Course/003 - f'
+    )
+  })
 })
 
 describe('metadata overrides (--replace-in-metadata)', () => {