Remove networking features in favor of next.ddr.tools - #617

The peer-to-peer send/sync features have been removed:

- deleted the peerjs store, peer menu, and shared-zustand sync plumbing
- dropped the share/sync actions from each drawn set, along with
  Drawing.__syncPeer and injectRemoteDrawing
- the networking settings tab now shows a short notice pointing at the
  next.ddr.tools alpha build
- the landing page callout advertises that alpha build for tournaments
  and live streams instead of local pack imports
- docs/networking.md now documents the discontinuation

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UbRGep2EybbANjMQTXA8m4

Reverts the code removal from the previous commit. The peer-to-peer
send/sync features work exactly as they did before; only the messaging
is new, so users get a heads up before anything disappears:

- the networking settings tab shows a warning callout above the peer
  controls noting the planned removal and linking to next.ddr.tools
- docs/networking.md keeps its full documentation behind a deprecation
  notice, and no longer promises future improvements to a design that
  is being retired
- docs/readme.md mentions the planned removal in its networking section
- the landing page callout advertises the alpha build for tournaments
  and live streams

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UbRGep2EybbANjMQTXA8m4