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

studio/backend/core/inference/llama_cpp.py

@@ -446,6 +446,13 @@ class LlamaCppBackend:
         Parses only the KV pairs we need (~30ms even for multi-GB files).
         For split GGUFs, metadata is always in shard 1.
         """
+        # Reset metadata from any previously loaded model so stale flags
+        # (eg _supports_reasoning) do not carry over when switching models.
+        self._context_length = None
+        self._chat_template = None
+        self._supports_reasoning = False
+        self._supports_tools = False
+
         try:
             WANTED = {"general.architecture", "tokenizer.chat_template"}
             arch = None