160 fix user login

[MaHaWo]

• fix runes mode usage
• make use of runes wherever appropriate
• activate biome linter rules
• fix the uncovered quality issues as far as possible, with 2 exceptions:
  • Flowbite-svelte components apparently don´t play nice with svelte 5 when being passed as arguments to other components and being used with out <svelte:component ... />
  • $locale is flagged as possibly null | undefined when using it as index variable, but I don´t see how this can happen in the current system.
• delete AbstractContent component that is no longer needed
• put tests for table functionality in separate file. this might go soon, but I'm hesitant to delete it.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 9.56072% with 350 lines in your changes missing coverage. Please review.

Project coverage is 29.99%. Comparing base (e4ad2fc) to head (f4cb0a7).

Files with missing lines	Patch %	Lines
...src/lib/components/DataDisplay/TableDisplay.svelte	0.00%	39 Missing and 1 partial :warning:
frontend/src/lib/components/MilestoneGroup.svelte	0.00%	40 Missing :warning:
frontend/src/lib/stores/userStore.svelte.ts	0.00%	34 Missing and 1 partial :warning:
frontend/src/lib/components/UserDataInput.svelte	0.00%	29 Missing :warning:
...end/src/lib/components/ChildrenRegistration.svelte	0.00%	24 Missing :warning:
frontend/src/lib/components/ChildrenGallery.svelte	0.00%	20 Missing :warning:
...c/lib/components/DataDisplay/GalleryDisplay.svelte	0.00%	19 Missing and 1 partial :warning:
...ontend/src/lib/components/MilestoneOverview.svelte	0.00%	19 Missing :warning:
frontend/src/lib/components/UserLogin.svelte	0.00%	15 Missing and 1 partial :warning:
frontend/src/lib/components/UserProfile.svelte	0.00%	11 Missing and 1 partial :warning:
... and 26 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #171 +/- ## ========================================== - Coverage 31.32% 29.99% -1.33% ========================================== Files 101 99 -2 Lines 3339 3247 -92 Branches 94 87 -7 ========================================== - Hits 1046 974 -72 + Misses 2221 2201 -20 Partials 72 72 ```

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[sonarcloud[bot]]

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

[MaHaWo]

thanks for the hints. will merge when ready.