Unify user-facing output: replace bare print() in library code with logger

[tschm]

Problem

Library code in src/pycharting/api/interface.py mixes two output channels for user-facing messages: it logs via logger and writes directly to stdout with bare print(), e.g. interface.py:284–285:

if _active_server and _active_server.is_running:
    logger.info("Stopping ChartServer...")
    _active_server.stop_server()
    print("✓ Chart server stopped")
else:
    print("ⓘ No active server to stop")

Bare print() in library code writes to a caller's stdout unconditionally (no level control, not capturable through logging config), which is surprising for a library consumed in scripts and notebooks.

Proposed fix

Route user-facing messages through a single channel — either the existing logger, or one small notify helper — instead of bare print(). Audit src/pycharting/ for any other bare print() calls and convert them too.

Acceptance criteria

• No bare print() remains in src/pycharting/.
• User-facing messages go through one consistent channel.
• Existing behavior is preserved and make test passes (update any tests that assert on captured stdout).

Subcategory: Code structure & readability (8 → 9) from the Rhiza quality assessment.