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/web/templates/partials/sidebar.html

@@ -27,6 +27,12 @@
   </div>
   {% endif %}
 
+  {# Every 10s, refresh the unread dots and announce anything that finished
+     while this page was showing something else. Out-of-band spans only, so the
+     folder tree keeps its open/closed state. #}
+  <div hidden hx-get="/api/chats/unread" hx-trigger="every 10s"
+       hx-swap="none"></div>
+
   <nav class="sidebar__scroll" id="sidebar-tree" aria-label="Chats">
     {% if pinned_models and can.get("chat.create") %}
     {# Shortcuts to start a chat with a particular model. These link rather than