feat: update reference sheet and refine character design prompt (#20)

## Summary

- Updates the reference image to the latest version
- Removes the barefoot/no socks instruction from the character prompt
- Clarifies that vampire fangs are only visible when the mouth is open

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #20
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

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