clock: add {month_name} placeholder with standalone month support

[yars068]

Support for POSIX %OB modifier of strftime() for clock/calendar module through a new placeholder {month_name}

What does this PR do?
Introduce a new {month_name} placeholder for the clock module tooltip. Unlike the standard library formatting which wraps month names in a date context (often forcing genitive case in inflected languages like Slavic, Baltic, or Finno-Ugric), this placeholder utilizes the POSIX %OB modifier to retrieve the standalone (nominative) month name.

Related issues
Closes #3890.

Checklist

• Code is formatted with clang-format
• Builds locally (ninja -C build)
• Man page updated for any new/changed user-facing option (man/)
• Tested against the affected module(s)

Distribuiton: Arch Linux, Hyprland 0.56.2.