Add All option to Format dropdown on events page

[dd32]

Summary

• The Format filter on the events page is a single-select dropdown (radio buttons) with no way to reset back to showing all formats
• Adds an "All" option as the default selection, consistent with how the Patterns directory handles its single-select filters
• When no format is actively filtered, "All" is pre-selected; selecting "Online" or "In Person" filters as before
• The empty value for "All" is safely handled by array_filter() in get_query_var_facets(), so it has no effect on the query

Test plan

• Visit the events listing page and open the Format dropdown
• Verify "All" is listed as the first option and is selected by default
• Select "Online", apply — verify only online events show
• Re-open Format, select "All", apply — verify all events show again
• Select "In Person", apply — verify only in-person events show

Fixes WordPress#1292

🤖 Generated with Claude Code

[dd32]

Filed upstream: WordPress#1669