feat: rebrand app and packaging icons to new Motionity logo

Swap the small icon mark, README banner, packaged app icon, and
favicon over to the new brand assets. make-icon.cjs and the new
make-favicon.cjs re-derive their bar geometry/colours from
src/assets/logo.svg instead of the old white-bars mark.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/assets/logo.svg

@@ -1,5 +1 @@
-<svg width="24" height="19" viewBox="0 0 24 19" fill="none" xmlns="http://www.w3.org/2000/svg">
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-<rect x="8.95996" width="6.08" height="19" rx="3.04" fill="white"/>
-<rect x="0.599609" width="6.08" height="19" rx="3.04" fill="white"/>
-</svg>
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