frontend: standardize base styles and color scheme, rework chat input section

frontend/src/components/Message.vue

@@ -43,8 +43,7 @@ function chatEntryClass(msg) {
     return ['log'];
   } else if (msg.type == 'thread-header') {
     let cls = ['thread'];
-    if (!msg.active) cls.push('thread-open')
-    if (msg.isFork) cls.push('thread-fork');
+    if (!msg.active) cls.push(msg.isFork ? 'thread-fork' : 'thread-closed')
     return cls;
   } else {
     return ['info'];
@@ -247,22 +246,18 @@ watch(() => [props.message.format, props.message.content], ([newFormat, newConte
   width: 100%;
   text-align: center;
   user-select: none;
-  border-top: 1px solid var(--color-border);
 }
 
-.thread.thread-open {
+.thread:hover {
+  border-color: 1px solid var(--color-text);
+}
+
+.thread-closed {
+  border-bottom: 1px solid var(--color-border);
   cursor: pointer;
 }
 
-.thread-open:hover {
-  border-top: 1px solid var(--color-heading);
-}
-
-.thread.thread-fork {
-  border-top: none;
-}
-
-.thread.thread-fork.thread-open {
+.thread-fork {
   width: fit-content;
   border: 1px solid var(--color-border);
   border-radius: var(--border-radius);