docs: picker folders and Video Player playlists

Pick mode now enters a folder on a tap and selects it on a long press, and
confirming without a selection returns the current folder; document that in
FileExplorerActivity. The Video Player turns a folder into a playlist, so the
formats page says so too.

Author: ThomasFarstrike

docs/apps/video-player.md

@@ -23,6 +23,12 @@ There is no H.264/H.265 or MPEG decode on the badge, and no scaling: frames are
 
 The picker opens at `/sdcard/video` and falls back to `/sdcard/clips`, then `/sdcard`. Any clip whose name ends in `.avi`, `.mjpeg` or `.mjpg` is listed. The video player is also registered as a viewer for those extensions, so "Open with" from the file manager works.
 
+### Folders and playlists
+
+Tapping a folder in the picker **enters** it, so nested folders are reachable. To play a folder as a playlist, long-press the folder to select it (or just press **Confirm** while inside it) and confirm: every clip in it plays in name order, which is why a naming scheme like `01_intro.avi`, `02_demo.avi` is worth using.
+
+When the last clip ends, playback stops and the player closes, unless `loop` was requested — then the folder starts over from the first clip. Each clip is started like a fresh file, so it gets its own frame rate from the container, its own soundtrack and its own timing report; a clip that cannot be played stops the playlist with its message rather than being skipped silently.
+
 ### Preparing clips
 
 Both formats are plain MJPEG, which every desktop player opens without extra codecs. With `ffmpeg`:

docs/frameworks/file-explorer-activity.md

@@ -9,7 +9,7 @@ The activity is registered for the `pick_file` action, so any app can ask the us
 Two modes are supported:
 
 - **`browse`** (default): navigate directories, open files, rename or delete them.
-- **`pick`**: select one or more files and return them to the caller. The `pick_file` action automatically uses pick mode, so callers do not need to set the `"mode"` extra.
+- **`pick`**: select one or more files and return them to the caller. The `pick_file` action automatically uses pick mode, so callers do not need to set the `"mode"` extra. Tapping a file toggles its selection; tapping a folder enters it (as in browse mode) and long-pressing a folder selects the whole folder.
 
 ## Launching
 
@@ -67,6 +67,17 @@ When the user confirms the selection, the result callback receives:
 }
 ```
 
+### Navigating and selecting in pick mode
+
+Pick mode browses folders exactly like the File Manager:
+
+- **Tap a folder** to enter it. The listing follows, so a nested folder can be reached without leaving the picker.
+- **Tap a file** to toggle its selection, so several files can be picked at once.
+- **Long-press a file or folder** to toggle the selection the same way. On a folder this picks the folder itself rather than entering it, and the release that follows the long press does not navigate.
+- **Confirm** without a selection returns the current folder.
+
+A folder in the result is only a path; what the caller does with it is up to the app. The Video Player, for example, plays every clip it contains in name order.
+
 If no file is selected, the current directory path is returned instead:
 
 ```python