Background generation, unread replies, send/stop, PLAN.md

**Replies now run in the background.** Generation was driven by the SSE
request, so navigating away or opening another chat cut the answer off
mid-sentence. services/generation.py owns the work as its own task and
the SSE endpoint merely follows it. Verified: attached briefly, closed
the connection, went to another page -- the reply finished anyway, 832
characters, not marked stopped, auto-titled.

Reattaching works because both `render` and `reasoning` frames now carry
the whole block rather than a delta. A follower arriving late has no
earlier fragments to append to, so deltas would leave it permanently
missing the beginning. Verified: attached six seconds in and the first
frame already contained 517 characters written while nobody watched.

**Unread indicator.** A reply that lands with no follower attached marks
its chat unread; the sidebar polls every 10s for out-of-band dot spans
plus an HX-Trigger that raises a toast. Polled rather than pushed: a
browser sitting on another chat has no connection to the one that
finished, and an always-on channel per tab is a lot of machinery for a
green dot. `unread_notified` stops the same arrival being announced
every tick. Follower count is what decides "was anyone watching", so
reading it as it arrives does not mark it unread -- verified both ways.

**Stop is the send button.** While a reply is being written the send
button becomes a red stop square, found via a MutationObserver on the
thread since the composer and the streaming bubble are far apart in the
document. The in-bubble Stop is gone.

**Attachment border removed.** As asked -- an attachment is a picture,
and the frame only ever drew at the wrong width. The anchor now
shrink-wraps and the img's width/height attributes are overridden so a
small image shows at its own size.

Adds PLAN.md: what is built, what is not, known limits, and the
decisions that look like oversights until you know the reason.

239 tests, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lembas/api/pages.py

@@ -117,6 +117,12 @@ async def chat_detail(request: Request, db: Db, user: RequiredUser, chat_id: str
     if chat is None or chat.user_id != user.id:
         raise HTTPException(status.HTTP_404_NOT_FOUND, "That chat no longer exists.")
 
+    # Opening the chat is what "read" means.
+    if chat.unread:
+        chat.unread = False
+        chat.unread_notified = False
+        db.commit()
+
     messages = list(
         db.scalars(
             select(Message).where(Message.chat_id == chat.id).order_by(Message.created_at)