Frontend: Improve icon contrast

[CezaryDanielNowak]

Currently icons on the thumbnail overlay (favourite, checkbox, etc) are white-ish.
Quite often it's invisible when photo is also light color. This makes it impossible see if photo is liked or selected.

Example:

After this change icons have dark shadow so it's more contrasting

Other affected icons:

Acceptance Criteria:

• Features and enhancements must be fully implemented so that they can be released at any time without additional work
• Automated unit and/or acceptance tests are mandatory to ensure the changes work as expected and to reduce repetitive manual work
• Frontend components must be responsive to work and look properly on phones, tablets, and desktop computers; you must have tested them on all major browsers and different devices
• Documentation and translation updates should be provided if needed
• In case you submit database-related changes, they must be tested and compatible with SQLite 3 and MariaDB 10.5.12+

[CLAassistant]

All committers have signed the CLA.

[CezaryDanielNowak]

double shadow for icons with opacity: 0.5

[lastzero]

Thank you! Before we merge it as it is, how do other applications handle it? Are there any other CSS tricks we could try? I haven't had time to compare it with e.g. Google Photos, but I don't remember seeing any shadows there.

[CezaryDanielNowak]

@lastzero Google Photos adds a shadow to whole image. If controls are on top: to the top. If controls are on the bottom - to the bottom.

Image zooms out when it's selected. I kind of like it, it's easier to distinguish if image is selected or not.