fix(serveui): make forced auto-scroll immediate

Force-scrolling the chat view now clears any pending delayed scroll and
updates the position right away instead of waiting for the throttle window.
This keeps manual "scroll to bottom" actions responsive while preserving
the normal auto-scroll behavior for regular updates.

Add a regression test covering the forced path and its interaction with an
already scheduled trailing scroll.

Author: SamSaffron

internal/serveui/static/app-core.js

@@ -740,9 +740,11 @@ const performScrollToBottom = (force = false) => {
 const scrollToBottom = (force = false) => {
   if (!elements.chatScroll) return;
   if (force) {
-    state.autoScroll = true;
+    clearPendingScrollToBottom();
+    performScrollToBottom(true);
+    return;
   }
-  if (!(force || state.autoScroll)) {
+  if (!state.autoScroll) {
     clearPendingScrollToBottom();
     return;
   }

internal/serveui/static/app_core_test.js

@@ -551,6 +551,58 @@ const app = loadAppCore();
   pass(name);
 })();
 
+(function testForceScrollBypassesThrottle() {
+  const name = 'force scroll bypasses throttle delay';
+  let nowMs = 1000;
+  let clearedTimer = 0;
+  const timers = [];
+  const chatScroll = Object.assign(makeNode(), {
+    scrollTop: 0,
+    scrollHeight: 1000,
+    clientHeight: 100,
+  });
+  const testApp = loadAppCoreWith({
+    nodeOverrides: { chatScroll },
+    now: () => nowMs,
+    timerOverrides: {
+      setTimeout(fn, delay) {
+        timers.push({ fn, delay });
+        return timers.length;
+      },
+      clearTimeout(id) {
+        clearedTimer = id;
+      },
+    },
+  });
+
+  testApp.state.autoScroll = true;
+  testApp.scrollToBottom();
+  nowMs = 1100;
+  chatScroll.scrollHeight = 1100;
+  testApp.scrollToBottom();
+  if (chatScroll.scrollTop !== 1000 || timers.length !== 1) {
+    fail(name, 'expected non-forced scroll to be throttled before forcing', JSON.stringify({ scrollTop: chatScroll.scrollTop, timers: timers.length }));
+    return;
+  }
+
+  chatScroll.scrollHeight = 1200;
+  testApp.scrollToBottom(true);
+  if (clearedTimer !== 1) {
+    fail(name, `expected forced scroll to clear pending trailing timer, got ${clearedTimer}`);
+    return;
+  }
+  if (chatScroll.scrollTop !== 1200) {
+    fail(name, `expected forced scroll to bottom immediately, got ${chatScroll.scrollTop}`);
+    return;
+  }
+  if (testApp.state.autoScroll !== true) {
+    fail(name, 'forced scroll should restore autoScroll');
+    return;
+  }
+
+  pass(name);
+})();
+
 (function testPendingScrollDoesNotFightUserScrollIntent() {
   const name = 'pending scroll does not fight user scroll intent';
   let nowMs = 1000;