feat(ui): plain-text user messages with 8-line auto-collapse

Pasted prompts (test cases, structured asks, JSON dumps) currently render
through the same markdown pipeline as assistant output, so '#'/'-'/'**'
characters are processed and long prompts dominate the scrollback.

- New UserMessageContent.vue: plain-text rendering (white-space: pre-wrap
  preserves user-typed newlines and indentation), with auto-collapse beyond
  8 lines and a "Show more (N more lines) / Show less" toggle. Soft mask
  gradient at the collapse boundary instead of a hard cut.
- MessageBubble.vue: route role==='user' messages through the new component;
  assistant messages keep the existing markdown pipeline unchanged.
- Add chat.expandLines / chat.collapse i18n keys (zh + en).

Verified end-to-end in browser preview: 15-line content collapses to 8,
toggle expands to full 15 with "Show less" label, raw '#' / '**' / '`' chars
shown literally with no <strong>/<h1>/<li> tags emitted.

mateclaw-ui/src/components/chat/MessageBubble.vue

@@ -124,8 +124,16 @@
           class="msg-content"
           :class="{ 'with-cursor': showCursor }"
         >
-          <div class="markdown-body" v-html="renderedContent"></div>
-          <TypingCursor v-if="showCursor" :typing="isGenerating" />
+          <!--
+            User-authored messages render as plain text (no markdown) and
+            auto-collapse beyond 8 lines. Assistant content goes through the
+            normal markdown pipeline.
+          -->
+          <UserMessageContent v-if="role === 'user'" :content="displayContent" />
+          <template v-else>
+            <div class="markdown-body" v-html="renderedContent"></div>
+            <TypingCursor v-if="showCursor" :typing="isGenerating" />
+          </template>
         </div>
 
 
@@ -272,6 +280,7 @@ import ToolCallSegment from './ToolCallSegment.vue'
 import ThinkingSegment from './ThinkingSegment.vue'
 import ContentSegment from './ContentSegment.vue'
 import PlanStepsPanel from './PlanStepsPanel.vue'
+import UserMessageContent from './UserMessageContent.vue'
 import type { BrowserAction } from './BrowserTimeline.vue'
 import type { Message, MessageSegment, ChatAttachment, ToolCallMeta, PlanMeta } from '@/types'
 import type { ChatErrorInfo } from '@/types/chatError'

mateclaw-ui/src/components/chat/UserMessageContent.vue

@@ -0,0 +1,118 @@
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { ArrowDown, ArrowUp } from '@element-plus/icons-vue'
+
+/**
+ * Renders a user-authored message as plain text (no markdown processing) and
+ * auto-collapses content longer than {@link COLLAPSE_LINE_THRESHOLD} lines.
+ *
+ * Why no markdown: user input is what the user typed, not a render directive.
+ * Markdown rendering surprises people who paste prompts containing #/`/-, and
+ * makes the bubble visually compete with assistant output (which IS markdown).
+ *
+ * Why 8 lines: long pasted prompts (test cases, structured asks, JSON dumps)
+ * dominate the scrollback otherwise. 8 lines fits the typical 1-2 paragraph
+ * ask without truncating.
+ */
+
+const props = defineProps<{
+  content: string
+}>()
+
+const { t } = useI18n()
+
+const COLLAPSE_LINE_THRESHOLD = 8
+
+const lines = computed(() => (props.content ?? '').split('\n'))
+const lineCount = computed(() => lines.value.length)
+const needsCollapse = computed(() => lineCount.value > COLLAPSE_LINE_THRESHOLD)
+
+const expanded = ref(false)
+
+const displayText = computed(() => {
+  if (!needsCollapse.value || expanded.value) return props.content ?? ''
+  return lines.value.slice(0, COLLAPSE_LINE_THRESHOLD).join('\n')
+})
+
+function toggle() {
+  expanded.value = !expanded.value
+}
+</script>
+
+<template>
+  <div class="user-msg-plain">
+    <div class="user-msg-plain__text" :class="{ 'is-collapsed': needsCollapse && !expanded }">{{ displayText }}</div>
+
+    <button
+      v-if="needsCollapse"
+      type="button"
+      class="user-msg-plain__toggle"
+      @click="toggle"
+    >
+      <span class="user-msg-plain__toggle-label">
+        {{ expanded ? t('chat.collapse') : t('chat.expandLines', { hidden: lineCount - COLLAPSE_LINE_THRESHOLD }) }}
+      </span>
+      <el-icon class="user-msg-plain__toggle-icon">
+        <component :is="expanded ? ArrowUp : ArrowDown" />
+      </el-icon>
+    </button>
+  </div>
+</template>
+
+<style scoped>
+.user-msg-plain {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+/*
+ * white-space: pre-wrap preserves the user's newlines and indentation
+ * (important for pasted prompts with structure) while still wrapping at the
+ * bubble's right edge. word-break:break-word keeps long URLs / paths from
+ * overflowing.
+ */
+.user-msg-plain__text {
+  white-space: pre-wrap;
+  word-break: break-word;
+  font-family: inherit;
+  line-height: 1.6;
+  color: inherit;
+  overflow-wrap: anywhere;
+}
+
+.user-msg-plain__text.is-collapsed {
+  /*
+   * Soft bottom fade so the collapse boundary doesn't look like a hard cut.
+   * Uses currentColor so the gradient inherits whatever bubble color is in
+   * effect (works on the orange user-bubble and on dark mode alike).
+   */
+  position: relative;
+  -webkit-mask-image: linear-gradient(to bottom, currentColor 70%, rgba(0, 0, 0, 0.35) 100%);
+  mask-image: linear-gradient(to bottom, currentColor 70%, rgba(0, 0, 0, 0.35) 100%);
+}
+
+.user-msg-plain__toggle {
+  align-self: flex-end;
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  padding: 2px 8px;
+  border: none;
+  background: rgba(255, 255, 255, 0.18);
+  color: inherit;
+  border-radius: 999px;
+  font-size: 12px;
+  cursor: pointer;
+  transition: background 0.15s ease;
+}
+
+.user-msg-plain__toggle:hover {
+  background: rgba(255, 255, 255, 0.28);
+}
+
+.user-msg-plain__toggle-icon {
+  font-size: 12px;
+}
+</style>

mateclaw-ui/src/i18n/locales/en-US.ts

@@ -53,6 +53,8 @@ export default {
     thinkingInProgress: 'Thinking...',
     stopped: 'Generation stopped',
     interrupted: 'Interrupted',
+    expandLines: 'Show more ({hidden} more lines)',
+    collapse: 'Show less',
     failed: 'Generation failed',
     compressionSummary: 'Previous conversations summarized',
     compressionWithCount: '{count} previous messages summarized',

mateclaw-ui/src/i18n/locales/zh-CN.ts

@@ -53,6 +53,8 @@ export default {
     thinkingInProgress: '思考中...',
     stopped: '已停止生成',
     interrupted: '已中断',
+    expandLines: '展开（还有 {hidden} 行）',
+    collapse: '收起',
     failed: '生成失败',
     compressionSummary: '之前的对话已整理为摘要',
     compressionWithCount: '之前的 {count} 条对话已整理为摘要',