Live Markdown, stop, rewind, custom picker, dialogs

Seven things.

**Reasoning starts closed.** The answer is what the reader is waiting
for; the thinking is one click away.

**Image borders.** .attachments__image was a block-level <a>, so its
border stretched the full column around a narrow picture. inline-block,
and the frame is the picture. Same fix for the composer thumbnail.

**Markdown now renders during the stream.** The generator re-renders the
answer so far and sends it as a `render` event at most every 100ms,
swapped with innerHTML, instead of appending escaped tokens and
formatting everything at the end. Re-rendering whole rather than
appending is the point: a list or a code fence is only correct once its
context exists, and partial syntax resolves itself as more arrives.
Measured against a live model: 29 render events, formatting visible from
the first content token.

**Stop button.** A stop request goes into an in-process set the
generator checks between chunks; whatever arrived is kept, because a
half-written answer the reader chose to cut short is still worth having.
Measured: stream ended 0.2s after the request, 1155 characters
preserved, message marked stopped rather than errored. Navigating away
does the same thing via CancelledError.

**Rewind and edit.** Edit one of your own turns and everything after it
is deleted, then the conversation runs on from there. Deliberately not
branching: that needs a UI for choosing between versions, and "go back
and try again from here" is what was asked for. The form states how many
messages will be discarded before you confirm.

**Custom model picker.** A <select> renders only text in an <option>, so
it can never show an avatar. Built from buttons and a hidden input, with
descriptions, capability tags, a filter box past eight models, and
arrow-key navigation written out by hand since there is no native widget
doing it.

**Notification system.** lembas.notify/confirm/prompt in ui.js, built on
<dialog> so focus trapping, Escape and page inertness come from the
browser. htmx:confirm is intercepted, so every existing hx-confirm gets
the themed dialog with no change at the call site; the browser's grey
confirm() is gone from every template.

230 tests, ruff clean.

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

README.md

@@ -35,9 +35,12 @@ runtime. Clone it, `pip install -e .`, run it.
   the first message, so an abandoned one never clutters the sidebar
 - **System prompts** — instance-wide, per-model and per-chat, with the most
   specific winning outright
-- **Reasoning display** — thinking from reasoning models streams into its own
-  collapsible block, labelled with how long it took, and is never replayed as
+- **Reasoning display** — thinking streams into its own collapsible block
+  (closed by default), labelled with how long it took, and is never replayed as
   context
+- **Live Markdown** — formatting appears as the model writes, not at the end
+- **Stop and rewind** — cut a reply short and keep what arrived, or edit an
+  earlier message and run the conversation on from there
 - **Attachments** — drag, paste or pick images, PDFs and text files. Images are
   downscaled and sent to vision models; PDF and text content is extracted and
   put in the prompt