Add configurable default chroma subsampling mode for JPEG encoder

[DT-art1]

Description

This PR adds support for configuring the default chroma subsampling mode used by the JPEG encoder.
Up until now, the encoder has always used H2V2 (4:2:0) subsampling internally.
For projects that benefit from higher chroma detail (e.g., thermal imaging, graphics, UI elements), it is useful to be able to select 4:2:2 or 4:4:4 instead.

What’s Added

• chroma_t enum defining available chroma subsampling modes:

  • CHROMA_444 (4:4:4)
  • CHROMA_422 (4:2:2)
  • CHROMA_420 (4:2:0)

• jpgSetChroma(chroma_t chroma) to set the global default encoder mode.

• The encoder now uses default_subsampling instead of a hardcoded subsampling value.
  The existing default behavior (H2V2 / CHROMA_420) remains unchanged unless the user calls jpgSetChroma().

Example Usage

jpgSetChroma(CHROMA_444);  // Use full chroma resolution (higher quality)

Related

Testing

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.