feat: add campus heilbronn events widget

[NicoStil101]

Changes:

• Add HeilbronnEventsWidgetView to the Campus screen, with a HeilbronnEventCardView for each event (title, date, time, location, link)
• Implement HeilbronnEvent model that parses each RSS into an event, reading title, link, description, publish date, start/end date, time, and location from their respective feed elements
• Implement HeilbronnEventService for the feed: selects the german feed (/de/events/index.xml) or english feed based on the app's current language
• Gate the widget on location: only fetch and show events when the user is within ~100 km of campus heilbronn, using a fresh GPS fix via LocationService
• Filter out past events (anything before today) and sort upcoming events by date ascending
• Trigger the fetch from didChangeDependencies so the current locale can be read safely, and refetch when the app language changes
• Add error handling with a retry action, and de/en translation strings

Testing:

• flutter analyze: new new issues
• tested on both iPhone and iPad
• no existing logic changed
• feed structure verified live against hn.fs.tum.de (EN and DE feeds)

Closes #351

Best regards,
TUM Fachschaft Heilbronn