kasuti: stage 3 replay — client-side GIF export

Adds a "Save GIF" button to the replay panel. On click, the component
renders each step of the replay to an offscreen canvas (both fabric
panels side-by-side, FRONT/BACK labels, active-side border, dashed
pattern guides for undrawn edges, drawn stitches in thread colour,
dashed start marker, red current-vertex dot) and feeds the frames
through gifenc — dynamically imported so the ~8 KB chunk only loads
when the user actually exports.

Frame timing: 200 ms between stitches, 900 ms hold on the first and
last frames. Palette is quantised to 64 colours per frame, which keeps
a Snowflake tour (~53 frames at 684×376) around 150–250 KB. The
button flips to "Encoding…" while busy and a sonner toast reports the
file size on success or the error on failure.

https://claude.ai/code/session_01KNdXjQczMCRGMK7K3Qderw

package.json

@@ -38,6 +38,7 @@
     "embla-carousel-autoplay": "^8.5.2",
     "embla-carousel-react": "^8.6.0",
     "framer-motion": "^11.18.2",
+    "gifenc": "^1.0.3",
     "gray-matter": "^4.0.3",
     "input-otp": "^1.4.2",
     "katex": "^0.16.45",