An installed app had no title bar of its own, no way to save attachments
except a zip, and "pop out" stayed inside the page. A new window was a
second copy of the app that had to restore the session itself, and on
start-up that evicts any login made without "Remember me" from the list
every window shares.
- Title bar: `display_override` with `window-controls-overlay`, one
`--titlebar-inset` variable at the page roots and skeletons, and a drag
strip with the window title. 0 without an overlay, so nothing moves.
- Save all to a folder: `showDirectoryPicker`, asking the device policy
before the picker and before each file, with the zip as the fallback.
- Pop-out routes `/popout/message/<id>` and `/popout/compose`, on offer in
an installed app or when the desktop bridge has windows. Hidden in Lite.
- A pop-out borrows the main window's session and never evicts or starts a
token timer. Every renewal now takes a Web Lock, so two windows can't
send the same refresh token, which Stalwart 1.0 may treat as a replay.
- A draft moves to its window under the same id. A closing window hands
unsaved text to the main window to save, since its own request wouldn't
survive the close.
- Read state, drafts, theme and language follow across windows, and
`html[data-desktop]` gates the app-like behaviour in `app/desktop.css`.
Linus Rath