fix: chat bubbles crushed to 6px — flex-shrink: 0 prevents row collapse

overflow: hidden on .chat-row made min-height resolve to 0 (CSS Flexbox §4.5),
so the flex column container crushed rows instead of scrolling. Also fix all
test selectors (.chat-agent→.chat-row-bot .chat-msg) and add bubble dimension
assertions.

static/css/04-interactive/_chat.css

@@ -330,14 +330,10 @@
     align-items: flex-start;
     max-width: 100%;
     overflow: hidden;
-}
-
-.chat-row-bot {
-    align-self: flex-start;
+    flex-shrink: 0;
 }
 
 .chat-row-user {
-    align-self: flex-end;
     justify-content: flex-end;
 }