Add site login and alternate-source search for paywalled articles

Deliberately not a paywall-bypass mechanism -- that space (archive.today-style
proxies, "Bypass Paywalls" extensions) is legally grey, adversarial toward
publishers, and constantly breaking. Instead:

- BrowserScreen: a real interactive mini-browser (JS + DOM storage on,
  ordinary navigation, no Readability interception) for the two cases where
  the user needs to actually USE a page rather than read it. Reached from
  a new overflow menu on the article reader:
  - "Log in to this site" opens the article's own URL there so the user can
    sign into a site they already subscribe to. Needs no extra plumbing:
    Android's CookieManager is one process-wide store shared by every WebView
    in the app, so a session cookie set here is picked up automatically by
    the reader's (JS-off) extraction WebView on the next visit to that
    domain -- same as a normal browser remembering you're signed in.
  - "Find another source" runs a web search on the article's title in the
    same browser, so the user can pick a different, non-paywalled outlet
    covering the same story themselves -- plain search/discovery, not
    circumvention.

Verified live: search opened a real, interactive Google results page with
working title-bar sync; back navigation (via BackHandler, goBack() when the
WebView has history, else pop the screen) returned cleanly to the article.

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