Add max_dim_for_mode to dimensions (missed in earlier commit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dimensions.py

@@ -2,6 +2,9 @@ MAX_OUTPUT_DIM = 2048
 MAX_OUTPUT_DIM_FAST = 768
 
 
+def max_dim_for_mode(mode): return MAX_OUTPUT_DIM_FAST if mode == "fast" else MAX_OUTPUT_DIM
+
+
 def compute_output_dimensions(w, h, max_dim=MAX_OUTPUT_DIM):
     # Pin the long side to max_dim and scale the short side proportionally.
     # We snap to the nearest multiple of 8 (not floor) to minimise aspect ratio