Integrate milestones userland with backend

[MaHaWo]

• integrate milestone and milestone overview into user landingpage
• remove old dummy code for milestones
• adjust Milestone component to the userland scheme (make use of setup function and #await block etc)
• include the categorization according to milestone groups again. Displaying all milestones could potentially become a lot of content, given that there are ~200 milestones that belong to < 10 age categories. This can be changed according to experience or desire by removing the MilestoneGroup component from the navigation breadcrumbs and adjusting the contentStore. I would ask for feedback from client in this regard.
• breadcrumbs and tabs can now have symbols that are used when the screen becomes too small for written labels to be sensibly displayed.
• add progress status to MilestoneGroupPublic which is computed from the currently active answer session. Since this is only an indicator for the user, it is computed on the fly and there is no additional field in the database model for it
• make the layout of the milestone such that it auto adjusts to viewport width
• store data of current child
• store current milestonegroup
• refactoring:
  • make more use of localization and svelte 5 runes mode
  • adjust breadcrumbs
  • add navigation button for milestonegroups to child data overview instead of relying on the breadcrumbs for it

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 354 lines in your changes missing coverage. Please review.

Project coverage is 30.01%. Comparing base (1496a3a) to head (05605a0). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
frontend/src/lib/components/MilestoneGroup.svelte	0.00%	95 Missing and 1 partial :warning:
...ontend/src/lib/components/MilestoneOverview.svelte	0.00%	87 Missing and 1 partial :warning:
frontend/src/lib/components/Milestone.svelte	0.00%	84 Missing :warning:
frontend/src/lib/stores/childrenStore.svelte.ts	0.00%	27 Missing and 1 partial :warning:
frontend/src/lib/stores/contentStore.svelte.ts	0.00%	23 Missing and 1 partial :warning:
...end/src/lib/components/ChildrenRegistration.svelte	0.00%	16 Missing :warning:
frontend/src/lib/components/ChildrenGallery.svelte	0.00%	6 Missing :warning:
...d/src/lib/components/Navigation/Breadcrumbs.svelte	0.00%	5 Missing and 1 partial :warning:
frontend/src/lib/components/UserLandingPage.svelte	0.00%	5 Missing :warning:
.../src/lib/components/DataDisplay/CardDisplay.svelte	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #158 +/- ## ========================================== + Coverage 27.85% 30.01% +2.15% ========================================== Files 96 93 -3 Lines 3461 3212 -249 Branches 93 90 -3 ========================================== Hits 964 964 + Misses 2426 2180 -246 + Partials 71 68 -3 ```

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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]

@lkeegan you want to have another look?

[MaHaWo]

would come back to this later to not stall this.