Make the floating AI chat window open at a larger default size and
allow stretching it further, plus shrink the fonts.
- ai-chat-window.tsx: DEFAULT_WIDTH 362->540, DEFAULT_HEIGHT 602->680;
clamp the default width to the viewport in computeInitialGeom()
(symmetric with the existing height clamp) to avoid overflow on
narrow screens.
- ai-chat-window.module.css: raise resize caps (max-width 560->900px,
max-height 880->1100px); base font-size 12->11px.
- ai-chat.module.css: chat content font .messages sm->xs.
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