Pinned models that know which side you are on

Reported: a pinned model always opened an ordinary chat, even with Agents
selected in the sidebar. They now carry `&kind=agent` with the switch -- a
preselection like `?model=` itself, so the new-chat screen still decides and
nothing is fixed until the first message is sent.

They sit above the tree the switch swaps, so this is the same shape as the New
chat button a few commits ago and gets the same treatment: their own partial,
arriving out of band. The group is rendered even when nothing is pinned,
because a block that vanished when the last model was unpinned would leave that
fragment with nowhere to land -- and htmx says nothing at all when a target is
missing, which is the silent failure this codebase keeps cataloguing.
`.nav-group--pinned:empty` stops the empty one taking room.

Chasing it turned up something else. The shortcuts came from `_chat_context`,
which only the chat pages build -- so the library, connections, settings and
folder pages carried the sidebar without them. A shortcut that is there on one
page and gone on the next. They come from `sidebar_context` now, where they
belong: it is sidebar content, and it is what the fragment route has.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/static/css/app.css

@@ -923,6 +923,10 @@ body.is-resizing .canvas__body { pointer-events: none; }
 /* --- Sidebar navigation ---------------------------------------------------- */
 .nav-group { margin-bottom: var(--sp-4); }
 .nav-group:last-child { margin-bottom: 0; }
+/* The pinned group is rendered even with nothing in it, so the Chat/Agent
+   switch has a target to swap out of band -- htmx says nothing when a target is
+   missing. Empty, it must take no room. */
+.nav-group--pinned:empty { display: none; margin: 0; }
 .nav-group__label {
   display: flex;
   align-items: center;