fix(hub,node): Create Group wizard silently skipped apps, and lost track of scanning progress

Two real-world bugs found together while testing multi-root group
creation:

- CreateGroupWizard only sent the enabled-apps PUT when the operator had
  *unchecked* something, assuming "every box left checked" already matched
  the node's own default (Roster.DEFAULT_APPS = chat, files). It doesn't —
  so leaving every app checked, the common case, silently left
  Videos/Music/Photos disabled on the node. Now sent unconditionally.

- The wizard's "add extra roots" step never polled index-status, so once
  step 3 (which only watches the first/upload root) finished, the
  progress bar froze while the node kept scanning the remaining roots for
  minutes, unwatched. Added waitForRootsIndexed (platform.js), mirroring
  waitForGroupHosted's own race handling.

That fix exposed a deeper one: indexer.py's _scan_root() only flipped
`progress.scanning` on *after* walking the directory and stat()-ing every
file — both off-loop, but slow enough on a large root that a poller's
grace period (waitForRootsIndexed's 5s) could expire before ever
observing `scanning: true` (confirmed against production logs: a GEK-init
step fired 5.058s after a root started scanning, matching the grace
period almost exactly). The stat() pass was also a synchronous loop
directly on the asyncio event loop — blocking the whole daemon (WebRTC,
chat, admin UI) for as long as it took on a root with many files. Both
fixed: `scanning` now flips on before the walk starts, and stat()-ing is
now off-loop too (_size_files).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013XSohfUQQiaE77qyFLgSv3

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