Add direct NavigaTUM and room calendar links to room details

[Copilot]

Room search exposed the relevant room details, but still required leaving the app to reach key NavigaTUM capabilities. This adds direct entry points from the room detail screen to both the canonical room page on nav.tum.de and the room calendar when the backend provides one.

• Room detail actions

  • Adds an app bar action to open the current room directly in NavigaTUM.
  • Adds a second action for the room calendar when props.calendar_url is available.

• NavigaTUM detail model

  • Extends room detail parsing to read props.calendar_url.
  • Adds derived URI helpers for:
    • the canonical room URL on nav.tum.de
    • the optional room calendar URL

• URL safety

  • Treats room IDs as opaque path segments when building the NavigaTUM URL.
  • Only exposes calendar links when they resolve to trusted HTTPS campus.tum.de URLs.

• Focused coverage

  • Extends NavigaTUM JSON model tests to cover:
    • calendar_url decoding
    • room URL construction
    • rejection of unsafe calendar URI schemes

Uri get navigaTumUri =>
    Uri(scheme: "https", host: "nav.tum.de", pathSegments: ["room", id]);

Uri? get calendarUri {
  final uri = additionalProperties.calendarUrl != null
      ? Uri.tryParse(additionalProperties.calendarUrl!)
      : null;
  if (uri == null || uri.scheme != "https" || uri.host != "campus.tum.de") {
    return null;
  }
  return uri;
}

• Fixes [Feature] Add link to the room calendar and nav.tum.de #357