Improve UX of join requests and fix minor bugs (#492)

## Description of the implemented changes
The changes were:
- [x] Bugfixing
- [x] New Feature
- [ ] Breaking Change
- [ ] Refactoring

This PR improves the join-request flow and presentation quality, fixes several data-display issues in join/join-request screens, and adds a usability improvement in global settings (directly opening the join link). It also includes dependency updates and changelog maintenance.

## What has been changed?
- Join form (`JoinLive`) now renders inputs based on actual field types (including checkbox/date/number/email behavior instead of generic text-only handling).
- Join form custom-field labels are resolved from configured custom fields (fallback remains safe if lookup fails).
- Join-request details page (`JoinRequestLive.Show`) now:
  - resolves and shows custom field names instead of raw IDs,
  - formats boolean-like values (`on/true/1`, `off/false/0`) as localized `Yes/No`,
  - formats ISO date strings for better readability,
  - keeps legacy field handling while improving output consistency.
- Join-request detail layout was improved semantically and visually (`dl/dt/dd` structure for label/value rows).
- Global settings page now includes an **Open** button for the join URL (`target="_blank"`, `rel="noopener noreferrer"`, ARIA label).
- Added/updated tests around:
  - join field type rendering,
  - custom field labels in join-request views,
  - related auth/global-settings behavior.
- Updated translations (`default.pot`, `en`, `de`) for new UI strings.
- Updated dependencies/tooling (`mix.lock`, `mix.exs`, CI/renovate-related updates).
- Updated `CHANGELOG.md` entries for unreleased changes.

## Definition of Done
### Code Quality
- [x] No new technical depths
- [x] Linting passed
- [x] Documentation is added were needed

### Accessibility
- [x] New elements are properly defined with html-tags
- [x] Colour contrast follows WCAG criteria
- [x] Aria labels are added when needed
- [x] Everything is accessible by keyboard
- [x] Tab-Order is comprehensible
- [x] All interactive elements have a visible focus

### Testing
- [x] Tests for new code are written
- [ ] All tests pass
- [ ] axe-core dev tools show no critical or major issues

## Additional Notes
- Reviewer focus areas:
  - `lib/mv_web/live/join_live.ex`: input type derivation and custom field lookup strategy (`authorize?: false` read path used intentionally for field metadata).
  - `lib/mv_web/live/join_request_live/show.ex`: value-formatting logic (especially backward compatibility for legacy `form_data` payloads).
  - `lib/mv_web/live/global_settings_live.ex`: external-link behavior and accessibility attributes.
- The branch also contains dependency update commits; please review lockfile and CI-related changes separately from functional join/join-request changes.

Reviewed-on: #492
Co-authored-by: Simon <s.thiessen@local-it.org>
Co-committed-by: Simon <s.thiessen@local-it.org>

lib/mv_web/live/global_settings_live.ex

@@ -186,6 +186,16 @@ defmodule MvWeb.GlobalSettingsLive do
                   <.icon name="hero-clipboard-document" class="size-4" />
                   {gettext("Copy")}
                 </.button>
+                <.link
+                  href={@join_url}
+                  target="_blank"
+                  rel="noopener noreferrer"
+                  class="btn btn-secondary btn-sm"
+                >
+                  <.icon name="hero-arrow-top-right-on-square" class="size-4" aria-hidden="true" />
+                  {pgettext("action", "Open")}
+                  <span class="sr-only">{gettext("join page URL in a new tab")}</span>
+                </.link>
               </div>
             </div>