[ psic4t/nospeak ] fix(chat): guard effect to prevent spurious message reloads
Add initialLoadDone flag to prevent the message loading effect from
re-running when stores emit without the partner actually changing.
This fixes the scroll jump/flicker that occurred ~3 seconds after
entering a chat due to background profile refresh triggering
Svelte reactivity.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>