fix: auto-grow textarea using CSS field-sizing: content

Replace JS-based auto-resize with native CSS field-sizing: content.
Eliminates timing issues with absolute-positioned containers.

frontend/src/components/transcript-debug/UserInput.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed, watch, nextTick } from 'vue'
+import { ref, computed, watch } from 'vue'
 import VoiceMicButton from './VoiceMicButton.vue'
 
 const props = defineProps<{
@@ -14,8 +14,7 @@ const props = defineProps<{
 
 const maxH = computed(() => {
   const lines = props.maxLines ?? 6
-  // line-height is 1.5 at 13px = ~20px per line, plus padding
-  return lines <= 1 ? '20px' : `${lines * 20}px`
+  return lines <= 1 ? '1.5em' : `${lines * 1.5}em`
 })
 
 const emit = defineEmits<{
@@ -25,14 +24,6 @@ const emit = defineEmits<{
 }>()
 
 const input = ref('')
-const textareaRef = ref<HTMLTextAreaElement | null>(null)
-
-function adjustHeight() {
-  const el = textareaRef.value
-  if (!el) return
-  el.style.height = 'auto'
-  el.style.height = el.scrollHeight + 'px'
-}
 
 const notReady = computed(() => props.terminalReady === false)
 const isDisabled = computed(() => !input.value.trim() || props.processing || notReady.value)
@@ -42,7 +33,6 @@ function handleSend() {
   if (!msg || props.processing || notReady.value) return
   emit('send', msg)
   input.value = ''
-  nextTick(adjustHeight)
 }
 
 function handleKeydown(e: KeyboardEvent) {
@@ -58,9 +48,6 @@ watch(() => props.voiceTranscript, (newText) => {
     input.value = newText
   }
 })
-
-// Auto-grow textarea when content changes
-watch(input, () => nextTick(adjustHeight))
 </script>
 
 <template>
@@ -79,7 +66,6 @@ watch(input, () => nextTick(adjustHeight))
     </div>
     <div class="input-container" :class="{ disabled: processing || notReady }">
       <textarea
-        ref="textareaRef"
         v-model="input"
         class="input-field"
         :style="{ maxHeight: maxH }"
@@ -182,7 +168,8 @@ watch(input, () => nextTick(adjustHeight))
   font-size: 13px;
   line-height: 1.5;
   resize: none;
-  min-height: 20px;
+  field-sizing: content;
+  min-height: 1lh;
   overflow-y: auto;
   padding: 0.15rem 0.25rem;
   font-family: inherit;