fix(scraper): push cookie, pageId and authUser to the scraper as one snapshot

[Me-xico]

What & why

Follow-up to #36 / maxrave-dev/SimpMusic#2505. That fix added authUser next to pageId,
but the three session values still reached the scraper through three independent DataStore
collectors, and Ytmusic stored them as three plain vars read one by one in ytClient().
A request built right after an account switch could carry a new pageId with the previous
authuser or cookie, for a short window.

• DataStoreManager.youTubeSession: one flow over one Preferences snapshot
  (distinctUntilChanged, since data re-emits on every unrelated preference).
• CommonRepositoryImpl: one collector calling YouTube.setSession(...) first, synchronously.
  visitorData is refreshed only when the cookie changes (same trigger as before, tracked
  with lastCookie, committed only on a successful fetch); the netscape cookie file is
  written once per session change (was twice).
• Ytmusic: @Volatile private var session: Session swapped as one immutable object;
  ytClient() and getAuthorizationHeader() read it once. cookie/pageId/authUser
  are read-only now, setSession is the only writer. The cached cookieMap is gone: the
  SAPISID map is parsed from the cookie actually sent (customCookie ?: session cookie).
• AccountRepositoryImpl.getAccountInfo no longer writes youTube.cookie directly: both
  callers already wrote DataStore and the request passes the cookie explicitly.
• Drive-by: getAuthorizationHeader() had if (...) null without return (no callers).

Not changed, worth knowing: parseCookieString throws on a ; -segment without =; it
now runs per request instead of inside the collector, so a bad pasted cookie no longer
kills the sibling jobs. A guard there is a separate follow-up.

Two pre-existing issues seen while testing, not touched here: every DataStore write
re-emits the proxy flows, removeProxy() sets proxy = null unconditionally and the
setter closes and recreates the HttpClient, so in-flight requests fail with
executor rejected (hit the visitorData() request on every account switch); and
visitorData() currently fails parsing sw.js_data with NoSuchElementException.
I can open issues for both.

Linked issue

Refs maxrave-dev/SimpMusic#2505

Checklist

• This PR addresses an accepted issue (linked above)
• I wrote or personally reviewed every line of this change
• I tested it on Android and/or Desktop — Desktop, run from source on Linux with two
  channels of the same Google account, watching the request log: startup (one session,
  X-Goog-Authuser: 0 + X-Goog-Pageid of the brand channel), switch to the main channel
  (one session, cookie file written once, next browse has no Pageid), switch back (Pageid
  back, never a mixed pair), logout (one session, next requests without Cookie or Pageid).
  Compiled scraper/domain/data for JVM and Android, :desktopApp:jvmMainClasses and
  :composeApp:compileAndroidMain. Not tested: re-login with the same cookie.