FR: Add 6:7 aspect ratio to crop module (e.g., Pentax 67)

Is your feature request related to a problem? Please describe.
The current list of aspect ratio presets in the crop module does not include the 6:7 ratio. This format is a standard in medium format photography, natively used by cameras such as the Pentax 67, Mamiya 7, and Mamiya RB67/RZ67.
I am currently using darktable 5.5.0~git3.69012abf-13518.1 and this ratio is still missing from the available presets.

Describe the solution you'd like
Add the 6:7 aspect ratio to the dropdown menu in the crop module.
Suggested label: 6:7 (e.g., Pentax 6x7)
Mathematical value: ~1.166 (resulting from 7/6)

Alternatives
Currently, there is no way to precisely define this ratio within the module's presets. The "freehand" option does not allow for a fixed ratio, and the "original image" option only works if the source file is already in 6:7. Users are forced to create a custom preset or manually edit the configuration, which is not an intuitive workflow for such a common photographic standard.

Additional context
As shown in the attached screenshot, the 6:7 ratio would logically be placed between "square" (1.00) and "10:8 in print" (1.20).

Start at darktable's crop module and locate the aspect-ratio preset dropdown and its existing ratio definitions. Add a fixed 6:7 entry in the position requested by the issue, then verify that it appears in the dropdown and preserves the ratio when cropping.