Fix bulk actions, create chats lazily, rework the UI

Seven reported problems.

**Bulk model actions 404'd.** /admin/models/{model_id} was registered
before /admin/models/bulk, and FastAPI matches in registration order, so
"bulk" was parsed as a model id. Moved above the parameterised route,
with a comment saying why, and a regression test.

**Empty chats piled up.** There is now no endpoint that creates one.
"New chat" is a link to /chat, which renders a composer with no row
behind it, and POST /api/chats/start writes the chat together with its
first message. Opening one and walking away leaves nothing.

**Pinning meant two different things.** The picker is now always in the
administrator's position order; pinned models get shortcuts in the chat
sidebar and nothing else. A picker whose order silently differs from the
admin screen is just confusing.

**Model images were missing in chat.** Assistant bubbles now show the
avatar of the model that actually wrote the turn -- which is not always
the model the chat is set to now -- falling back to the LLeMbas mark.
The picker shows it too.

**No global or per-model system prompt.** Three layers now: instance
(Admin -> General), model (Admin -> Models), chat. Precedence, not
concatenation: most specific wins outright. Stacking them reads well in
a settings screen and badly in practice, because two layers that
disagree give the model contradictory instructions and nobody can tell
which is losing. The chat panel shows the inherited prompt as
placeholder text so "leave empty to inherit" is not a guess.

**Alignment and button sizing.** Added --control-h and friends to
tokens.css; every button, input and select takes its height from them,
so a mixed row is flush by construction rather than by per-instance
nudging. Icon buttons are square at that height. Added .btn-row,
.card__header/.card__footer and .grid so pages stop carrying inline
styles, and moved every admin page onto them.

**Settings needed structure.** The user settings page is now tabbed
(Account / Models / Appearance / Security) using radio inputs and
sibling selectors -- no JavaScript, and the browser keeps the chosen tab
across a re-render.

Caught while checking: the chat.css surgery had deleted the attachment,
chip and dropzone rules. Restored, and there is now a check that every
literal class used in a template has a CSS rule.

197 tests, ruff clean.

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

src/lembas/api/admin_models.py

@@ -59,12 +59,43 @@ async def models_page(request: Request, db: Db, user: AdminUser, saved: str = ""
             "groups": list(db.scalars(select(Group).order_by(Group.name))),
             "connections": list(db.scalars(select(Connection).order_by(Connection.name))),
             "default_model": settings_store.get(db, "default_model") or "",
+            "instance_prompt": settings_store.get(db, "system_prompt") or "",
             "capabilities": CAPABILITIES,
             "saved": saved,
         },
     )
 
 
+# Registered BEFORE /{model_id}: FastAPI matches in registration order, so
+# with the parameterised route first, "bulk" is captured as a model id and
+# the handler 404s on a model that does not exist.
+@router.post("/admin/models/bulk")
+async def bulk_models(
+    db: Db, user: AdminUser, action: str = Form(...), model_ids: list[str] = Form(default=[])
+) -> Response:
+    """Enable or disable several models at once.
+
+    A freshly refreshed connection can advertise dozens of models; turning them
+    off one at a time is not a reasonable way to spend an afternoon.
+    """
+    models = list(db.scalars(select(Model).where(Model.id.in_(model_ids or []))))
+    for model in models:
+        if action == "enable":
+            model.enabled = True
+        elif action == "disable":
+            model.enabled = False
+        elif action == "public":
+            model.public = True
+            model.groups = []
+        elif action == "private":
+            model.public = False
+    db.commit()
+    _renumber(db)
+    return RedirectResponse(
+        f"/admin/models?saved={len(models)}+model(s)+updated.", status_code=303
+    )
+
+
 @router.post("/admin/models/{model_id}")
 async def update_model(
     db: Db,
@@ -72,6 +103,7 @@ async def update_model(
     model_id: str,
     display_name: str = Form(""),
     description: str = Form(""),
+    system_prompt: str = Form(""),
     enabled: bool = Form(False),
     pinned: bool = Form(False),
     public: bool = Form(False),
@@ -82,6 +114,7 @@ async def update_model(
 
     model.display_name = display_name.strip()[:300]
     model.description = description.strip()[:2000]
+    model.system_prompt = system_prompt.strip()[:8000]
     model.enabled = enabled
     model.pinned = pinned
     model.public = public
@@ -165,33 +198,6 @@ async def delete_model_image(db: Db, user: AdminUser, model_id: str) -> Response
     return RedirectResponse("/admin/models?saved=Image+removed.", status_code=303)
 
 
-@router.post("/admin/models/bulk")
-async def bulk_models(
-    db: Db, user: AdminUser, action: str = Form(...), model_ids: list[str] = Form(default=[])
-) -> Response:
-    """Enable or disable several models at once.
-
-    A freshly refreshed connection can advertise dozens of models; turning them
-    off one at a time is not a reasonable way to spend an afternoon.
-    """
-    models = list(db.scalars(select(Model).where(Model.id.in_(model_ids or []))))
-    for model in models:
-        if action == "enable":
-            model.enabled = True
-        elif action == "disable":
-            model.enabled = False
-        elif action == "public":
-            model.public = True
-            model.groups = []
-        elif action == "private":
-            model.public = False
-    db.commit()
-    _renumber(db)
-    return RedirectResponse(
-        f"/admin/models?saved={len(models)}+model(s)+updated.", status_code=303
-    )
-
-
 # --- Serving model images ----------------------------------------------------
 @router.get("/uploads/models/{filename}")
 async def model_image(user: RequiredUser, filename: str) -> Response: