Add tool confirmation UI with Accept/Decline for write operations

Before executing any write tool (create/update/delete), the backend now
pauses with an asyncio.Future and emits a tool_pending SSE event. The
frontend displays a ToolConfirmCard with Accept and Decline buttons.
Clicking Accept resolves the Future and proceeds; Decline records a
declined tool_call chip and falls through to regular streaming. Typing
single-word yes/no responses (e.g. "yes", "cancel") also works as
confirmation. 120s timeout auto-declines if no response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ToolCallCard.vue

@@ -11,6 +11,7 @@ const appliedTags = ref<Set<string>>(new Set());
 const applyingTag = ref<string | null>(null);
 
 const label = computed(() => {
+  if (props.toolCall.status === "declined") return "Declined";
   if (!props.toolCall.result.success) return "Error";
   switch (props.toolCall.result.type) {
     case "task":
@@ -180,9 +181,12 @@ async function applyTag(tag: string) {
 </script>
 
 <template>
-  <div class="tool-call-card" :class="{ error: toolCall.status === 'error' }">
+  <div class="tool-call-card" :class="{ error: toolCall.status === 'error', declined: toolCall.status === 'declined' }">
     <span class="tool-label">{{ label }}</span>
-    <template v-if="toolCall.status === 'error'">
+    <template v-if="toolCall.status === 'declined'">
+      <span class="tool-declined-name">{{ toolCall.arguments.title ?? toolCall.arguments.summary ?? toolCall.arguments.query ?? toolCall.function }}</span>
+    </template>
+    <template v-else-if="toolCall.status === 'error'">
       <span class="tool-error">{{ toolCall.result.error }}</span>
     </template>
     <template v-else-if="searchResults">
@@ -292,6 +296,13 @@ async function applyTag(tag: string) {
 .tool-call-card.error {
   border-color: var(--color-danger, #e74c3c);
 }
+.tool-call-card.declined {
+  opacity: 0.55;
+}
+.tool-declined-name {
+  text-decoration: line-through;
+  color: var(--color-text-muted);
+}
 .tool-label {
   font-weight: 600;
   color: var(--color-text-muted);