studio: fix stale GGUF metadata, update helper model, auth improvements (#4346)

* studio: switch helper model to Qwen3.5-4B-GGUF

Replace Qwen3-4B-Instruct-2507-GGUF with Qwen3.5-4B-GGUF as the
default helper model for LLM-assisted dataset detection. Same
UD-Q4_K_XL variant.

* studio: fix stale GGUF metadata when switching models (#4347)

Reset _supports_reasoning, _supports_tools, _context_length, and
_chat_template at the start of _read_gguf_metadata() to prevent
stale settings from a previous model leaking into the next load.

Co-authored-by: Daniel Han <daniel@unsloth.ai>

* studio: change login error to "Incorrect password", add reset-password CLI

- Login error now says "Incorrect password" instead of the generic
  "Incorrect username or password" since Studio only has one account.
- Add `unsloth studio reset-password` command that deletes the auth
  database so a fresh admin account with a new random password is
  created on the next server start.

* studio: include reset command in login error message

* studio: change password setup subtitle wording

cli/commands/studio.py

@@ -178,3 +178,28 @@ def setup():
 
     if result.returncode != 0:
         raise typer.Exit(result.returncode)
+
+
+# ── unsloth studio reset-password ────────────────────────────────────
+
+
+@studio_app.command("reset-password")
+def reset_password():
+    """Reset the Studio admin password.
+
+    Deletes the auth database so that a fresh admin account with a new
+    random password is created on the next server start.  The Studio
+    server must be restarted after running this command.
+    """
+    auth_dir = STUDIO_HOME / "auth"
+    db_file = auth_dir / "auth.db"
+    pw_file = auth_dir / ".bootstrap_password"
+
+    if not db_file.exists():
+        typer.echo("No auth database found -- nothing to reset.")
+        raise typer.Exit(0)
+
+    db_file.unlink(missing_ok = True)
+    pw_file.unlink(missing_ok = True)
+
+    typer.echo("Auth database deleted. Restart Unsloth Studio to get a new password.")