Improve timeslot forms with compact table layout

[Simrayz]

Scope and purpose

Fixes #1794.

Replaces the bulky stacked fieldset layout for time recurrences with a compact table-based layout. Also reworks the timeslot list to use accordion cards with lazy-loaded forms, adds flatpickr time pickers, and improves form UX with inline success/error banners.

This pull request

• adds flatpickr as a dependency for time inputs on the timeslot page

Contributor Checklist

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation, including updates to the user manual if feature flow or UI is considerably changed
• Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See our how-to
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
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

[github-actions]

Test results

    6 files    894 suites   1m 48s ⏱️
  701 tests   700 ✅ 1 💤 0 ❌
4 206 runs  4 200 ✅ 6 💤 0 ❌

Results for commit ecef61d.

[codecov]

Codecov Report

❌ Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.73%. Comparing base (d284743) to head (ecef61d).

Files with missing lines	Patch %	Lines
src/argus/htmx/timeslot/views.py	68.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1797      +/-   ##
==========================================
+ Coverage   82.55%   82.73%   +0.17%     
==========================================
  Files         137      137              
  Lines        6552     6573      +21     
==========================================
+ Hits         5409     5438      +29     
+ Misses       1143     1135       -8     

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