refactor: extract label positioning constants

[krystophny]

The axis label positioning uses hardcoded pixel values that should be extracted as named constants for better maintainability.

Locations

• src/fortplot_raster.f90 line 1054: py offset of 30 pixels
• src/fortplot_raster.f90 line 1069: px offset of 10 pixels
• Similar hardcoded values in tick positioning

Recommendation

Create positioning constants module or add to margins module:

• XLABEL_VERTICAL_OFFSET
• YLABEL_HORIZONTAL_OFFSET
• TICK_LABEL_OFFSET
• TICK_MARK_LENGTH

Found during code review of PR #341

[krystophny]

Fixed in PR #370. Label positioning constants extracted to shared module with descriptive names.