Add return-type annotations to stop_server() and _repr_html_() in interface.py

[tschm]

Problem

Two public functions in src/pycharting/api/interface.py lack explicit return-type annotations:

• stop_server() at interface.py:261 — should be -> None
• _repr_html_() at interface.py:325 — should be -> str

make typecheck (ty) passes today because ty infers these, but the functions would flag under a mypy --strict posture, and the public API surface is inconsistently annotated (get_server_status() -> dict[str, Any] is annotated, the others are not).

Proposed fix

Add the explicit return types:

def stop_server() -> None:
    ...

def _repr_html_() -> str:
    ...

Verify plot(...) (interface.py:35) is also fully annotated while in the file.

Acceptance criteria

• Every public function in interface.py has an explicit return-type annotation.
• make typecheck still passes.

Subcategory: Type safety (8 → 9) from the Rhiza quality assessment.