feat(assistant): deck-it's chat look on the canvas

The canvas takes the deck-it Assistant tab's look: the backdrop picture
faded in behind the chat through a soft elliptic mask (stronger while the
chat is empty), and chat bubbles, the question on the right in an accent
bubble with the user's avatar, the reply on the left with the assistant's
face cropped from the backdrop. Thinking, tool chips and errors sit in the
reply bubble; three dots bounce while it has no words yet.

The pictures come from deck-it's config ([assistant] background,
user_avatar, face), read once at startup with the system python3, so
both follow one setting. MultiEffect's mask needs maskSpreadAtMin for a
gradient to fade; the default thresholds cut it to a hard oval.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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