relay: consistent 16px gap between compose-sidebar sections on mobile

#compose-sidebar only had its flex-column + gap:16px declaration
inside the desktop media query, so on mobile the inbox/shared/sent
cards stacked as plain blocks with no deliberate spacing. Move the
rule to base CSS so the rhythm between inbox → shared → sent
matches the compose → inbox gap regardless of viewport width.
Desktop re-declares it next to the grid layout for clarity; the
duplication is harmless.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

relay/ui/style.css

@@ -232,6 +232,14 @@ label.checkbox input[type="checkbox"] { accent-color:var(--accent); cursor:point
 #view-compose:not(.hidden) {
     display: flex; flex-direction: column; gap: 16px;
 }
+/* Same 16 px rhythm between inbox / shared / sent on mobile as
+   between compose and inbox — desktop re-declares this inside the
+   grid rule further down. */
+#compose-sidebar {
+    display: flex;
+    flex-direction: column;
+    gap: 16px;
+}
 .compose-attachments {
     display: flex; flex-wrap: wrap; gap: 6px; min-height: 0;
 }