fix: cross-platform config locking (Windows support)

[jdwit]

Fixes #49.

Problem

Since the v0.2.0 profiles refactor, config.py does import fcntl at module top level. fcntl is Unix-only, so on Windows the import raises ModuleNotFoundError before any command can run. config.py is imported transitively by everything (main.py -> api.py -> config.py), so the entire CLI is unusable on Windows from v0.2.0 through v0.4.1. Last working release on Windows was v0.1.1.

Thanks to @leotulipan for the detailed report and root-cause analysis.

Fix (stdlib only, no new dependency)

• Import fcntl and msvcrt conditionally; either may be None.
• Route _config_lock() through _lock_file / _unlock_file helpers: fcntl.flock on POSIX, msvcrt.locking on Windows, graceful no-op if neither is available (acceptable for a single-user CLI).
• Guard chmod(0o700) in _ensure_private_dir behind os.name == "posix" (on Windows chmod only toggles the read-only bit, so the owner-only guarantee is POSIX-only by nature).
• The os.open(..., 0o600/0o644) mode bits in the atomic writers are already silently ignored on Windows, so they need no change.

Tests

Added TestCrossPlatformLock:

• lock round-trips on the host platform,
• with fcntl patched to None the lock routes to msvcrt.locking (LK_LOCK / LK_UNLCK, 1 byte),
• with neither primitive present the lock degrades to a no-op instead of crashing,
• a real set_active_profile mutation succeeds on the simulated-Windows path.

Full suite: 300 passed, ruff clean. (CI on a Windows runner would be the ideal follow-up to lock this in for good.)

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.28%. Comparing base (c4816e1) to head (636834a).

Files with missing lines	Patch %	Lines
src/ytstudio/config.py	87.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #51   +/-   ##
=======================================
  Coverage   84.28%   84.28%           
=======================================
  Files          15       15           
  Lines        2500     2520   +20     
  Branches      367      372    +5     
=======================================
+ Hits         2107     2124   +17     
- Misses        262      264    +2     
- Partials      131      132    +1     

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