feat(collaboration): live presence + session chat (cursors, mentions, emoji, pins)

[professionalarduino-max]

Adds Figma-style live presence and a session chat to live-collaboration sessions 🎉

1. Live presence (cursors & co.)

• src/lib/dash-presence.js — ephemeral presence channel: cursor (surface + position), open sprite, dragged block. Relayed over the collaboration WebSocket as best-effort presence messages — never written to the persistent Yjs document, so it also works for viewers.
• PresenceCursors overlay — colored cursors with name labels:
  • on the stage (normalized coordinates, resolution independent);
  • in the blocks workspace (workspace coordinates, so cursors stay glued to blocks across zoom/scroll differences);
  • remote blocks being dragged get a colored outline + a ⠿ badge on the cursor.
• Sprite indicators — tiles of sprites that other participants have open show colored dots and a ring.
• PresenceAvatars in the menu bar — who is in the session, who is currently online.

2. Session chat (src/lib/dash-chat.js + dash-chat panel)

• Chat panel in the collaboration status bar with message history (server keeps a 200-message room history, replayed on join).
• @username mentions — highlighted in messages, unread badge + tooltip notification when someone mentions you.
• Emoji picker — 6 categories, inserted at the caret.
• Pinned messages — pin a message to the current sprite / costume / block; pin chips navigate on click (block pins flash-highlight the block). Filter toggle for pinned-only.

Compatibility — feature handshake

• auth advertises features: ['presence', 'chat'], ready lists server support.
• Presence/chat are only sent and relayed when both sides support them: old clients never receive unknown message types (which used to kill their session), and new clients never send presence/chat to old servers (which used to close the connection).

Notes

• Requires the matching DashBlocks/server change (PR Bump scratch-vm from 555aa35 to 929071f #3 in DashBlocks/server).
• Presence throttled ~12 msg/s; presence payloads capped at 2 KB, chat text at 500 chars server-side; peers expire after 12 s.
• All changes follow the existing collab architecture (vm.dashCollaboration session, DASH_COLLABORATION_STATUS events).