fix: support grayscale images in fit_image padding

[cryptomilk]

ImageOps.pad and Image.new require a color matching the image's band count. This fixes fitting for single-band (e.g. grayscale "L") source images.

I'm working on https://github.com/cryptomilk/hass-eink-dashboard which creates grayscale images, it failed with a TypeError:

Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib/python3.14/site-packages/homeassistant/components/opendisplay/services.py", line 225, in _async_upload_image                   
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     await device.upload_image(                                                                                                                  
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     ...<6 lines>...                                                                                                                             
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     )                                                                                                                                           Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib/python3.14/site-packages/opendisplay/device.py", line 979, in upload_image                                                     
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     image_data, compressed_data, processed_image = self._prepare_image(                                                                         Jul 12 15:05:01 antares.milli.ways hass[1429759]:                                                    ~~~~~~~~~~~~~~~~~~~^                                                                         
Jul 12 15:05:01 antares.milli.ways hass[1429759]:         image,                                                                                                                                  Jul 12 15:05:01 antares.milli.ways hass[1429759]:         ^^^^^^                                                                                                                                  
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     ...<10 lines>...                                                                                                                            Jul 12 15:05:01 antares.milli.ways hass[1429759]:         rotate=rotate,                                                                                                                          
Jul 12 15:05:01 antares.milli.ways hass[1429759]:         ^^^^^^^^^^^^^^                                                                                                                          Jul 12 15:05:01 antares.milli.ways hass[1429759]:     )                                                                                                                                           
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     ^                                                                                                                                           
Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib/python3.14/site-packages/opendisplay/device.py", line 895, in _prepare_image                                                   
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     return prepare_image(                                                                                                                       
Jul 12 15:05:01 antares.milli.ways hass[1429759]:         image,                                                                                                                                  
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     ...<14 lines>...                                                                                                                            
Jul 12 15:05:01 antares.milli.ways hass[1429759]:         rotate=rotate,                                                                                                                          
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     )                                                                                                                                           
Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib/python3.14/site-packages/opendisplay/device.py", line 209, in prepare_image                                                    
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     image = fit_image(image, target_size, fit)                                                                                                  
Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib/python3.14/site-packages/opendisplay/encoding/images.py", line 38, in fit_image                                                
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     return ImageOps.pad(image, target_size, Image.Resampling.LANCZOS, color=_PAD_COLOR)                                                         
Jul 12 15:05:01 antares.milli.ways hass[1429759]:            ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib64/python3.14/site-packages/PIL/ImageOps.py", line 363, in pad
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     out = Image.new(image.mode, size, color)
Jul 12 15:05:01 antares.milli.ways hass[1429759]:   File "/usr/lib64/python3.14/site-packages/PIL/Image.py", line 3241, in new
Jul 12 15:05:01 antares.milli.ways hass[1429759]:     return im._new(core.fill(mode, size, color))
Jul 12 15:05:01 antares.milli.ways hass[1429759]:                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^
Jul 12 15:05:01 antares.milli.ways hass[1429759]: TypeError: color must be int or single-element tuple

[cryptomilk]

Tested and works.

[codecov]

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