Friendlier unsupported model errors, show estimated download size

1. Backend: When a model fails with "No config file found" or similar
   unsupported-model errors, wrap the message with "This model is not
   supported yet. Try a different model." instead of showing the raw
   Unsloth exception.

2. Frontend: Compute estimated download size from the HF search API's
   safetensors.parameters dtype breakdown (BF16=2B/param, I32=4B/param,
   F32=4B/param, etc.) and show it in the model picker instead of just
   the param count. For example, Kimi-K2.5 now shows "~554 GB" instead
   of "171B" (which was misleading since 171B params != 171GB download).

studio/frontend/src/components/assistant-ui/model-selector/pickers.tsx

@@ -408,8 +408,13 @@ export function HubModelPicker({
     () =>
       new Map(
         results
-          .filter((result) => result.totalParams)
-          .map((result) => [result.id, formatCompact(result.totalParams!)]),
+          .filter((result) => result.totalParams || result.estimatedSizeBytes)
+          .map((result) => [
+            result.id,
+            result.estimatedSizeBytes
+              ? `~${formatBytes(result.estimatedSizeBytes)}`
+              : formatCompact(result.totalParams!),
+          ]),
       ),
     [results],
   );