The level picker explains itself, as it does on iOS

This screen showed chips and no explanation; iOS has carried one all along,
under a comment claiming both apps use the same words. They now do.

The last sentence is the boundary: a level decides who can open a file, not
where it can be put. Neither app stops somebody filing sensitive material
somewhere low, and saying so is the honest half of android#15.

Also repairs a comment the emoji sweep truncated, which left the test
saying 'the -property is NOT enforced'.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/main/kotlin/io/flamenet/messenger/app/FilesScreen.kt

@@ -580,6 +580,20 @@ private fun ColumnScope.LevelPicker(
             )
         }
     }
+    Spacer(Modifier.height(Space.xs))
+    // Word for word what iOS says under its picker. The two apps must not
+    // describe the same rule differently, and this screen said nothing at all.
+    // The last sentence is the honest boundary: neither app stops somebody
+    // filing sensitive material somewhere low. See android#15.
+    Text(
+        "Who can open this. A folder's level doesn't limit what you put inside " +
+            "it, each file keeps its own. It is sealed with the label bound in, " +
+            "so this is chosen now rather than changed later. It decides who can " +
+            "open a file, not where the file can be put.",
+        fontSize = 12.sp,
+        color = Palette.SecondaryText,
+        modifier = Modifier.testTag("files-level-note"),
+    )
 }
 
 // ---- Into and out of a conversation --------------------------------------.