feat: update reference sheet and refine character design prompt

Updates the reference image to the latest version. Removes the barefoot
instruction from the character prompt and clarifies that vampire fangs
are only visible when the mouth is open.

src-tauri/src/gemini.rs

@@ -11,11 +11,11 @@ Character design (always required):
 - Wavy ashen brown hair (colour and texture fixed; hairstyle can vary)
 - Very pale skin tone
 - Vibrant sky-blue eyes — important, commonly missed
-- Vampire fangs
+- Vampire fangs (only visible when mouth is open)
 - Glasses (pink-framed preferred, other styles acceptable)
 - Painted fingernails and toenails (any colour, never unpolished)
 - Slender build
-- Full body visible in frame; always barefoot, never wears socks
+- Full body visible in frame
 
 Composition (always required):
 - Single character only