feat(kez-chat/web): auto-scroll thread on new message + on conversation open

Two rules, picked to feel natural:

1. Conversation just opened (peer_primary changed) → always scroll to
   bottom. You expect to see the latest exchange first.

2. New message landed in the current conversation → scroll to bottom
   IF you were already within 120px of the bottom. If you're scrolled
   up reading older history, the auto-scroll doesn't yank you back
   down. (Slack / Telegram / iMessage all do this; getting yanked out
   of history when you're searching for something is infuriating.)

Implementation: a single $effect tracks activeConv.messages.length +
activeConv.peer_primary. Compares against two cursor vars (prevPrimary,
prevMessageCount) to distinguish "opened a new conversation" from
"new message in current one". queueMicrotask after the DOM updates so
scrollHeight reflects the just-rendered message.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

kez-chat/web/src/routes/Messages.svelte

@@ -77,6 +77,50 @@
   /** Unsubscribe handle for the inbox-service "new message" listener. */
   let unsubscribe: (() => void) | null = null;
 
+  /** Scroll container for the thread view — bound for auto-scroll. */
+  let scrollEl: HTMLDivElement | null = $state(null);
+  /** Tracks the previously-rendered conversation so we know when the
+   *  user just OPENED a different thread (always scroll to bottom) vs.
+   *  when a new message landed in the current one (scroll only if they
+   *  were near the bottom already — don't yank them out of history). */
+  let prevPrimary: string | null = null;
+  let prevMessageCount = 0;
+
+  /** Distance (px) from the bottom of the thread within which we still
+   *  count "near bottom" for the follow-along rule. Roughly two short
+   *  bubble heights — generous enough that small misses don't sticky. */
+  const SCROLL_FOLLOW_THRESHOLD_PX = 120;
+
+  // Auto-scroll effect — runs whenever activeConv.messages.length or
+  // activeConv.peer_primary changes (both are reactive reads).
+  $effect(() => {
+    if (!activeConv || !scrollEl) {
+      prevPrimary = null;
+      prevMessageCount = 0;
+      return;
+    }
+    const el = scrollEl;
+    const count = activeConv.messages.length;
+    const conversationChanged = activeConv.peer_primary !== prevPrimary;
+    if (conversationChanged) {
+      prevPrimary = activeConv.peer_primary;
+      prevMessageCount = count;
+      // Wait a microtask so Svelte's DOM updates have flushed.
+      queueMicrotask(() => (el.scrollTop = el.scrollHeight));
+      return;
+    }
+    if (count > prevMessageCount) {
+      prevMessageCount = count;
+      const distFromBottom =
+        el.scrollHeight - el.scrollTop - el.clientHeight;
+      if (distFromBottom < SCROLL_FOLLOW_THRESHOLD_PX) {
+        queueMicrotask(() => (el.scrollTop = el.scrollHeight));
+      }
+    } else {
+      prevMessageCount = count;
+    }
+  });
+
   // "Start chat with" lookup state.
   let newPeerInput = $state("");
   let resolving = $state(false);
@@ -351,7 +395,7 @@
         </p>
       </div>
 
-      <div class="flex-1 overflow-y-auto p-4 space-y-2">
+      <div class="flex-1 overflow-y-auto p-4 space-y-2" bind:this={scrollEl}>
         {#each activeConv.messages as m (m.seq + ":" + m.direction)}
           {@const boost = emojiOnlyBoost(m.body)}
           <div class={`max-w-md ${m.direction === "out" ? "ml-auto" : ""}`}>