Switching folders currently keeps an active search applied and re-runs
it in the newly selected folder — deliberate behavior from #553, where
users wanted the query to follow them across folders. Others expect the
opposite: clicking Inbox/Sent should drop the search and just browse the
folder.
Add an opt-in setting (Settings → email behavior) that reconciles both.
Default is off, so #553's search-follows-you behavior is unchanged for
everyone. When on, any folder click clears the text query and advanced
filters and browses the folder instead of re-running the search.
Applied uniformly across the four folder-entry paths in MailApp — normal
mailboxes, per-account (Pro sidebar) folders, unified views, and cross
views — via one helper so no path diverges.