fix(studio): block element selection while composition is loading

Prevent users from selecting elements in the preview while the
composition is still loading (showing "Loading composition" overlay).
Selection and hover highlighting are suppressed until the player fires
the ready event.

Also reverts motion panel and manual drag editing defaults to false —
these were accidentally set to true during the PR #693 merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: miguel-heygen