Hold Enter (context menu) on the DM / Channels / Rooms picker opens a
single-item "Mark all read" menu that clears every unread counter of
the highlighted type. Per-mode title is a static-const string because
PopupMenu stores the title pointer verbatim.
New UITask::clearAllDMUnread() resets the whole _dm_unread_table;
clearAllChannelUnread() and clearRoomUnread() already existed.
Also document the GPS breadcrumb storage decision in FEATURES.md:
RAM-only ring with explicit per-slot saves, no background flash writes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
