fix: empty calendar

[limwa]

Fixes an issue where fetching the calendar returns no events due to a wrong URL, after conversion of %20 to + in Dart's standard lib.

Review checklist

• [x] Terms and conditions reflect the current change
• [ ] Contains enough appropriate tests
• [x] If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• [x] Properly adds an entry in changelog.md with the change
• [x] If PR includes UI updates/additions, its description has screenshots
• [x] Behavior is as expected
• [x] Clean, well-structured code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 11%. Comparing base (1fc65d3) to head (5515d1d). Report is 8 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1334 +/- ## ======================================= - Coverage 11% 11% -0% ======================================= Files 268 269 +1 Lines 7330 7341 +11 ======================================= Hits 802 802 - Misses 6528 6539 +11 ```