Fix nested route support for InboxLive

[chrisgreg]

Fix nested route support for InboxLive

Problem

When mounting the FYI routes in a nested scope (e.g., /admin/fyi instead of /fyi), the
InboxLive URLs were hardcoded to /fyi/*, causing 404 errors when clicking on event details.

Solution

• Dynamically extract the route prefix from the current URI in handle_params/3
• Updated build_url/3 and event_url/4 to use the extracted route prefix instead of
  hardcoded paths
• Made event_url/4 handle both socket and assigns parameters for template compatibility

Testing

• Added comprehensive test suite (test/fyi/web/inbox_live_test.exs) with 13 tests covering:
  • Route prefix extraction from various URI formats
  • URL generation for root-level and nested scopes
  • Event detail and index URLs with query parameters
• All 65 tests pass

Impact

Users can now mount FYI routes at any scope level:

• /fyi (default)
• /admin/fyi
• /admin/dashboard/fyi
• Any other nested scope