fix: config profile form focus loss and path validation messages

- Fix React key stability in env vars, files, and mount file inputs
  to prevent focus loss on every keystroke
- Improve validation error messages to explain Files vs Mounts
- Add helper text in UI clarifying relative vs absolute paths

Fixes focus loss bug and improves UX for path validation errors.

apps/api/src/api/config_profiles.py

@@ -78,8 +78,13 @@ class MountItem(BaseModel):
     @classmethod
     def validate_files(cls, v: dict) -> dict:
         for path in v.keys():
-            if ".." in path or path.startswith("/") or not path:
+            if ".." in path or not path:
                 raise ValueError(f"Invalid file path: {path}")
+            if path.startswith("/"):
+                raise ValueError(
+                    f"Mount file paths must be relative (got: {path}). "
+                    f"The mount target defines the absolute container path."
+                )
         return v
 
 
@@ -103,8 +108,13 @@ class ConfigProfileCreate(BaseModel):
     @classmethod
     def validate_files(cls, v: dict) -> dict:
         for path in v.keys():
-            if ".." in path or path.startswith("/") or not path:
+            if ".." in path or not path:
                 raise ValueError(f"Invalid file path: {path}")
+            if path.startswith("/"):
+                raise ValueError(
+                    f"File paths must be relative (got: {path}). "
+                    f"Use Mounts for absolute container paths."
+                )
         return v
 
     @field_validator("env_vars")