support rotation snapping

[fm-sys]

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Android 7 low-res screen emulator
• Android 15 Physical tablet

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

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Description

This PR implements feature request https://community.signalusers.org/t/74780. As it is a rather trivial feature and doesn't need synchronization between different platforms and differs already today (e.g., in signal desktop you can perform resize and rotate operations independently), I thought it might be a suitable feature for an external contribution.

When merged, rotation angles less than 5° get automatically snapped to the next 90° rotation step. Based on some testing on my side, 5° seemed like the sweet spot between usability for snapping and keeping as much freedom as possible to allow other creative angles.

This is important for all the perfectionists out there, who feel real physical pain if they can't align things properly, so please consider this :)

Screen_recording_20260528_123816.mp4