create r4-supabase-filter-search and use pages/base-channels

[4www]

• create r4-supabase-filter-search (for /explore and /:slug/tracks)
• try to use pages/base-channels

To be continued:

• make it works on /explore
• refactor /explore menu (so map and search can be moved there)
• continue the "building filters" components, so tags/mentions could be the same and other things too

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