Constrain the chat column height so the message stream scrolls

Why: .chat is a column-flex item of .main without min-height:0, so its
content-based automatic minimum height grew past the viewport; .chat-body
never overflowed (never scrolled) and the input bar was pushed off-screen.
Evidence: headless Chromium on mock serve, 1024x620: before .chat=2130px
and input-bar bottom=717 (>viewport), chat-body scrollable=false; after
.chat=620, chat-body clientH 501 / scrollH 2011 with wheel scroll moving
600px up and back to bottom, input-bar bottom=620. Variants (empty
conversation, single short message, 1024x400, 720x600) keep the input
visible. Full combo make test green.
Impact: chat page was unusable beyond one screen of content; single CSS
rule chain, no behavior change elsewhere.
Refs: Agent-System-Dev task card feature/chat-scroll-fix.

src/styles.css

@@ -39,10 +39,12 @@ button { font-family: inherit; }
 .icon-only button:hover { background: var(--bg-hover); color: var(--text); }
 
 /* ---- 中栏对话 ---- */
-.chat { flex: 1; display: flex; flex-direction: column; min-width: 0; }
+/* min-height:0 须随列向 flex 链下传（.main→.chat→.chat-body）：flex 项的自动最小
+   高度是内容高度，缺失时消息会撑破 .chat——chat-body 永不滚动、输入栏被顶出视口。 */
+.chat { flex: 1; display: flex; flex-direction: column; min-width: 0; min-height: 0; }
 .chat-header { padding: 10px 18px; border-bottom: 1px solid var(--border); display: flex; align-items: center; gap: 10px; background: var(--bg-raised); }
 .chat-title { font-weight: 600; flex: 1; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
-.chat-body { flex: 1; overflow-y: auto; padding: 18px max(18px, 12%); }
+.chat-body { flex: 1; overflow-y: auto; min-height: 0; padding: 18px max(18px, 12%); }
 .empty-state { height: 100%; display: flex; flex-direction: column; align-items: center; justify-content: center; color: var(--text-faint); gap: 8px; }
 .msg { margin-bottom: 18px; }
 .msg-role { font-size: 12px; color: var(--text-faint); margin-bottom: 4px; display: flex; gap: 8px; align-items: center; }