Working chat: auth, connections, streaming, folders

LLeMbas now runs end to end. Register, add an OpenAI-compatible
connection, and hold a real streaming conversation organised into
folders. Verified against the local llama-swap instance.

Streaming is the one genuinely tricky part. Sending a message returns
two HTML fragments -- the user bubble and an empty assistant bubble
carrying an sse-connect -- and that attribute is the ONLY thing that
starts a generation. Rendering an incomplete assistant message as a
streaming shell falls out of the same template, which means loading a
page whose last reply never finished simply picks it up again.

Details worth knowing about, each commented where it matters:

- SSE payloads are split across several data: lines. A raw newline in
  one data: line truncates the event, which shows up the first time a
  model emits a code block.
- Markdown is rendered server-side by the same helper for both the page
  and the final streamed frame, so the two cannot disagree. The fence
  renderer is replaced outright rather than using markdown-it's
  highlight option, which re-wraps output in a second <pre>.
- escape_text is html.escape, not nh3.clean_text: it escapes character
  by character, so escaping stream chunks separately equals escaping
  the whole string.
- The stream opens its own session via session_scope(); it outlives the
  request handler and the dependency-scoped session may be closed.
- Deleting a folder keeps the chats inside it (FK is SET NULL). Losing
  a conversation to a mis-clicked folder delete is unforgivable.
- Login failures use one message for "no such account" and "wrong
  password" so the form cannot enumerate registered addresses.

Also adds deploy/ for the gamebox install at https://chat.lan: system
unit, nginx vhost with buffering off (buffering on turns streaming into
one lump at the end), and install/update scripts following the same
service-user and /srv bind-mount conventions as llama-swap and comfyui.

70 tests, ruff clean.

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

src/lembas/web/templates/partials/_folder.html

@@ -0,0 +1,46 @@
+{% from "_macros.html" import icon %}
+{#
+  A folder and everything under it. Recursive: a folder includes this same
+  template for each of its children, so nesting has no fixed depth limit.
+
+  Open/closed state is local to the browser (Alpine), not persisted per
+  request -- collapsing a folder should not cost a round trip.
+#}
+<div class="folder" x-data="{ open: {{ 'false' if folder.collapsed else 'true' }} }">
+  <div class="nav-item folder__row">
+    <button class="folder__toggle" type="button" @click="open = !open"
+            :aria-expanded="open ? 'true' : 'false'">
+      <span class="folder__chevron" :class="open && 'is-open'">
+        {{ icon("chevron-right", "icon--sm") }}
+      </span>
+      <span x-show="open">{{ icon("folder-open", "icon--sm") }}</span>
+      <span x-show="!open" x-cloak>{{ icon("folder", "icon--sm") }}</span>
+      <span class="nav-item__label">{{ folder.name }}</span>
+    </button>
+    <span class="nav-item__actions">
+      <button class="btn btn--icon btn--sm" type="button"
+              hx-delete="/api/folders/{{ folder.id }}"
+              hx-confirm="Delete the folder “{{ folder.name }}”? Chats inside it are kept."
+              hx-swap="none"
+              aria-label="Delete folder">
+        {{ icon("trash", "icon--sm") }}
+      </button>
+    </span>
+  </div>
+
+  <div class="folder__contents" x-show="open" x-cloak>
+    {% for child in folder.children %}
+      {% with folder = child %}
+        {% include "partials/_folder.html" %}
+      {% endwith %}
+    {% endfor %}
+
+    {% for chat_item in folder.chats %}
+      {% include "partials/_chat_link.html" %}
+    {% endfor %}
+
+    {% if not folder.children and not folder.chats %}
+    <p class="nav-empty">Empty</p>
+    {% endif %}
+  </div>
+</div>