Add optional horizontal (landscape) layout mode

[kfezer]

Summary

• Adds a top-level "layout": "horizontal" setting to settings.json that keeps the display's native landscape orientation and tiles modules left-to-right instead of the default top-to-bottom stacking.
• Each module's size is then read as [column width, full height] instead of [full width, row height].
• Default behavior (layout absent, or set to anything other than "horizontal") is completely unchanged — this is purely additive, gated behind the new setting.
• No changes needed to individual module classes (Calendar, Xkcd, etc.) — they already just render into whatever box _assemble() gives them.

Related to the long-requested horizontal mode discussed in #117. That thread was closed pending a fuller 2-column grid rewrite ("would require a 2-column grid instead of one"). This PR is a smaller, pragmatic step that gets basic side-by-side layouts working today without that larger rewrite — happy to adjust direction if a fuller grid system is still preferred instead.

Test plan

• Verified on real hardware: 12.48" colour e-paper (epd_12_in_48_colour) on a Raspberry Pi Zero 2 W, with layout: horizontal, Calendar module on the left (size: [550, 964]) and Xkcd module on the right (size: [754, 964]). Both rendered correctly and matched the assigned columns.
• Confirmed default (no layout key) still produces the original vertical-stacked layout — the vertical code path is untouched, only reached via an added if/else branch.
• python3 -m py_compile inkycal/main.py passes.
• No new automated test added for _assemble() — the existing tests/test_main.py doesn't cover this method and I didn't want to bolt on test scaffolding without knowing your preferred fixture/mocking approach for module image composition. Happy to add one if you can point me at a pattern to follow.

Docs

Added a short section to docs/docs/modules.md explaining the new setting and showing an example.

[github-actions]

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