post view: bottom-anchor the post-line thread (flex column + justify-content: flex-end) so short threads sit above the composer instead of the header

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/static_src/scss/_room.scss

@@ -311,6 +311,9 @@ html.sea-open .room-aperture.is-scrollable {
             flex: 1;
             min-height: 0;
             overflow-y: auto;
+            display: flex;
+            flex-direction: column;
+            justify-content: flex-end;   // bottom-anchor short threads
 
             .post-line {
                 display: grid;