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/settingsSchema.test.ts

@@ -85,6 +85,20 @@ describe('parseSettingsField (CC9 schema parity)', () => {
     expect(parseSettingsField('filenameTemplate', '../%(title)s.%(ext)s')).toEqual(fail)
   })
 
+  it('collectionOutputTemplate: empty allowed; relative multi-segment ok; traversal rejected', () => {
+    expect(parseSettingsField('collectionOutputTemplate', '')).toEqual(ok(''))
+    expect(
+      parseSettingsField(
+        'collectionOutputTemplate',
+        ' %(uploader)s/%(playlist)s/%(title)s.%(ext)s '
+      )
+    ).toEqual(ok('%(uploader)s/%(playlist)s/%(title)s.%(ext)s'))
+    expect(parseSettingsField('collectionOutputTemplate', '../%(title)s.%(ext)s')).toEqual(fail)
+    expect(parseSettingsField('collectionOutputTemplate', 'C:\\abs\\%(title)s.%(ext)s')).toEqual(
+      fail
+    )
+  })
+
   it('rateLimit: yt-dlp format or empty', () => {
     expect(parseSettingsField('rateLimit', '2.5M')).toEqual(ok('2.5M'))
     expect(parseSettingsField('rateLimit', ' 500K ')).toEqual(ok('500K'))