feat(frontend): redesign post-login layout into guild/channel/chat panes

src/index.css

@@ -76,38 +76,64 @@ a:hover {
   text-decoration: underline;
 }
 
-.app-shell {
-  display: grid;
-  grid-template-columns: 240px 1fr;
-  min-height: 100vh;
-}
-
-.sidebar {
-  border-right: 1px solid var(--border);
-  padding: 20px 16px;
-  text-align: left;
-  background: color-mix(in srgb, var(--bg) 94%, var(--accent) 6%);
-}
-
-.sidebar-title {
-  margin: 0 0 14px;
-}
-
-.sidebar-nav {
-  display: grid;
-  gap: 10px;
-}
-
-.sidebar-session {
-  margin-top: 20px;
-  font-size: 13px;
-}
-
 .page-content {
   padding: 18px;
   text-align: left;
 }
 
+.chat-layout {
+  display: grid;
+  gap: 12px;
+}
+
+.chat-top-grid {
+  display: grid;
+  grid-template-columns: 220px 260px 1fr;
+  gap: 12px;
+  min-height: 68vh;
+}
+
+.chat-right {
+  display: grid;
+  grid-template-rows: 1fr auto;
+  gap: 12px;
+  min-height: 0;
+}
+
+.chat-history {
+  overflow: auto;
+}
+
+.composer {
+  display: flex;
+  gap: 8px;
+}
+
+.composer .input {
+  flex: 1;
+}
+
+.footer-actions {
+  display: flex;
+  gap: 8px;
+}
+
+.list-btn {
+  width: 100%;
+  text-align: left;
+  border: 1px solid var(--border);
+  background: transparent;
+  border-radius: 8px;
+  padding: 8px 10px;
+  color: var(--text-h);
+  cursor: pointer;
+}
+
+.list-btn.active {
+  border-color: var(--accent-border);
+  background: var(--accent-bg);
+}
+
 .panel {
   border: 1px solid var(--border);
   border-radius: 12px;