Search the whole folder tree, not just the folder being browsed

The query box filtered the current listing, which only ever answered is-it-here; the usual question is where-did-I-put-it. domain/FileSearch walks down from the current folder and streams matches as they are found, since a whole-volume walk takes tens of seconds and a blank screen for that long reads as a hang. Hits show where they were found, and the app bar shows progress while the walk runs.

Bounds, all deliberate: a matching folder is still descended into; a hidden folder is skipped whole rather than merely filtered, so the preference cannot be leaked around; the walk stops at 200 matches; each keystroke cancels the previous walk instead of queueing; and navigating away abandons the search.

CHANGELOG.md

@@ -38,6 +38,21 @@ upload needs a **higher `versionCode`** than every previous upload.
 
 ### Added
 
+- **Recursive search.** The query box used to filter the folder you were standing in, which
+  can only ever answer "is it here?" — and the usual question is "where did I put it?". It
+  now walks *down* from the current folder (`domain/FileSearch`) and streams matches as they
+  are found, because walking a whole volume takes tens of seconds and a blank screen for
+  that long reads as a hang. Rows say where the hit was found (`Folder · Documents/2026`) and
+  the app bar shows progress while the walk is still going.
+
+  Three deliberate bounds: a matching folder is still descended into, so a folder called
+  `holiday` cannot hide the matching files inside it; a hidden folder is skipped *whole*
+  rather than merely filtered out, so the preference cannot be leaked around; and the walk
+  stops at 200 matches, because a search returning ten thousand rows is not more useful than
+  one returning two hundred. Each keystroke cancels the walk in progress instead of queueing
+  behind it, and navigating away abandons the search — its results belonged to the folder
+  being left.
+
 - **Undo for delete, rename and move.** A delete is now a *move* into a hidden
   `.plainfiles-trash` folder on the same volume rather than a removal, so the snackbar can
   offer to put it back; a rename and a pasted cut record how to reverse themselves. Four