studio: reorder GGUF preference -- UD-Q4_K_XL first, all UD above standard

Reorder _GGUF_QUANT_PREFERENCE so all UD (Unsloth Dynamic) variants
come before standard quants. UD-Q4_K_XL is the default (best
size/quality tradeoff), followed by other UD quants in decreasing
preference order.

For repos without UD variants (e.g., bartowski), falls through to
standard quants starting with Q4_K_M.

Verified with:
  - unsloth/Qwen3.5-35B-A3B-GGUF -> UD-Q4_K_XL
  - bartowski/Qwen_Qwen3.5-35B-A3B-GGUF -> Q4_K_M
  - unsloth/DeepSeek-V3.2-GGUF -> UD-Q4_K_XL (9 shards)
  - unsloth/Llama-3.2-1B-Instruct-GGUF -> UD-Q4_K_XL

studio/backend/utils/models/model_config.py

@@ -801,8 +801,29 @@ def detect_gguf_model(path: str) -> Optional[str]:
 
 # Preferred GGUF quantization levels, in descending priority.
 # Q4_K_M is a good default: small, fast, acceptable quality.
+# UD (Unsloth Dynamic) variants are always preferred over standard quants
+# because they provide better quality per bit. If the repo has no UD variants
+# (e.g., bartowski repos), the standard quants are used as fallback.
+# Ordered by best size/quality tradeoff, not raw quality.
 _GGUF_QUANT_PREFERENCE = [
+    # UD variants (best quality per bit) -- Q4 is the sweet spot
     "UD-Q4_K_XL",
+    "UD-Q4_K_L",
+    "UD-Q5_K_XL",
+    "UD-Q3_K_XL",
+    "UD-Q6_K_XL",
+    "UD-Q6_K_S",
+    "UD-Q8_K_XL",
+    "UD-Q2_K_XL",
+    "UD-IQ4_NL",
+    "UD-IQ4_XS",
+    "UD-IQ3_S",
+    "UD-IQ3_XXS",
+    "UD-IQ2_M",
+    "UD-IQ2_XXS",
+    "UD-IQ1_M",
+    "UD-IQ1_S",
+    # Standard quants (fallback for non-Unsloth repos)
     "Q4_K_M",
     "Q4_K_S",
     "Q5_K_M",
@@ -811,11 +832,15 @@ _GGUF_QUANT_PREFERENCE = [
     "Q8_0",
     "Q3_K_M",
     "Q3_K_L",
+    "Q3_K_S",
     "Q2_K",
-    "UD-Q2_K_XL",
-    "UD-IQ2_M",
-    "UD-IQ1_M",
-    "UD-IQ1_S",
+    "Q2_K_L",
+    "IQ4_NL",
+    "IQ4_XS",
+    "IQ3_M",
+    "IQ3_XXS",
+    "IQ2_M",
+    "IQ1_M",
     "F16",
     "BF16",
     "F32",