Feature/preset temporary visibility

[Just1Developer]

Temporary Visibility of Presets

This update adds functionality such that presets can be visible only in a certain time frame.

Features

• Presets are loaded all the same, but have two new fields: visibleFrom and visibleUntil. This is a Date that is represented by YYYY-MM-DD, and if the current local date is outside of the given bounds for a preset, it doesn't show up.
  If a bound should be ignored, you can leave it empty, or enter 0-0-0. Both parse to 0000-00-00, which is ignored in the bounds check.

Fixes

• Fixed a bug where the semester would be incorrectly set to the next year when opening an existing file while its January-March. January 2026 should be mapped to WS25/26, but was instead mapped to WS26/27. I'm pretty sure this is the third time I've fixed this issue, but it still appears. Someday, I will rewrite the entire UI.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Just1Developer]

@robinmaisch @uuqjz

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