An instance that can be somebody else's

A name, a tagline, a logo, a favicon and the launcher icons derived from it; the
Middle-earth strings as data; themes as token sets; and a stylesheet for what
none of that reaches. All four are on one page, in one settings group.

The snapshot is a Jinja global over a process-level cache, because render() has
no session and four render paths never reach it at all -- the sign-in page, the
error pages, the offline page and the SSE fragments. A context value would have
had to be threaded through every one and would still have missed those. It being
a global is also what lets mark() branch on an uploaded logo without any of its
six call sites learning about branding; the macro that renders the sidebar link
is called brandlink now, because a macro imported as `brand` shadows the global
for the whole template and took out every page at once.

Defaults in code and overrides in the database, as the prompt fragments do, with
one difference stated in the module: an empty fragment means off, an empty
flavour string means the shipped wording. And blanked rather than dropped --
settings_store.update merges, so an omitted key leaves what was stored last time
and "I typed the default back in" would store something different from "I changed
nothing".

A custom theme sets a handful of tokens and inherits the rest, and the
inheritance is a CSS fact: tokens.css matches [data-base="shire"] as well as
[data-theme="shire"], so a custom light theme lands on parchment rather than four
light colours on near-black. Values are validated on read rather than on save,
because a theme written straight into the settings table still has to produce a
stylesheet that parses -- a `}` in a value ends the rule and silently breaks
every rule after it. The soft variants are derived from the accent, or a changed
accent leaves focus rings in the old hue and reads as half-working.

/branding.css is a route, not an inline block: an external stylesheet has no HTML
context to escape from. The link carries a content hash, so a save is not left to
the browser's cache, and it is deliberately outside the service worker's precache
list, which is versioned by the release.

The instance name moved off /admin/general rather than being duplicated there.
An upgrade keeps it: the general row is read as a seed exactly while the branding
row has never mentioned the name, which is `key in row` and not `row[key] is
truthy` -- the two read alike would resurrect the old name underneath a cleared
one.

The theme list stops being a hard-coded pair in five places. Every failure mode
in that area is silent, so it is driven under a DOM stub as well as tested.

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

src/lembas/main.py

@@ -16,6 +16,7 @@ from lembas.api import (
     admin,
     admin_agents,
     admin_audio,
+    admin_branding,
     admin_images,
     admin_models,
     admin_prompts,
@@ -27,6 +28,7 @@ from lembas.api import (
     agents,
     audio,
     auth,
+    branding,
     canvas,
     chats,
     files,
@@ -181,6 +183,7 @@ def create_app() -> FastAPI:
     app.include_router(admin_users.router)
     app.include_router(admin_models.router)
     app.include_router(admin_audio.router)
+    app.include_router(admin_branding.router)
     app.include_router(admin_search.router)
     app.include_router(admin_schedules.router)
     app.include_router(admin_images.router)
@@ -189,6 +192,7 @@ def create_app() -> FastAPI:
     app.include_router(admin_tools.router)
     app.include_router(admin_agents.router)
     app.include_router(push.router)
+    app.include_router(branding.router)
 
     register_error_handlers(app)
     return app
@@ -219,7 +223,7 @@ def register_error_handlers(app: FastAPI) -> None:
             {
                 "status_code": exc.status_code,
                 "detail": exc.detail,
-                "flavour": ERROR_FLAVOUR.get(exc.status_code, ERROR_FLAVOUR[500]),
+                "flavour": error_flavour(exc.status_code),
             },
             status_code=exc.status_code,
         )
@@ -233,18 +237,24 @@ def register_error_handlers(app: FastAPI) -> None:
             request,
             "error.html",
             {"status_code": 500, "detail": "Something went wrong.",
-             "flavour": ERROR_FLAVOUR[500]},
+             "flavour": error_flavour(500)},
             status_code=500,
         )
 
 
 # Flavour lives in error pages, empty states and theme names -- never in the
 # functional UI. See CLAUDE.md.
-ERROR_FLAVOUR = {
-    403: "Speak, friend, and enter. This door is not yours to open.",
-    404: "Not all those who wander are lost. This page, however, is.",
-    500: "The Road goes ever on, but this stretch of it has washed out.",
-}
+#
+# The three lines themselves moved into `services/branding.py` with the rest of
+# what an administrator can replace. What is left here is the mapping from a
+# status code to which of them, which is not something anybody would want to
+# edit. `snapshot()` never raises, so an error page can still render its error
+# on an instance whose database is the thing that broke.
+def error_flavour(status_code: int) -> str:
+    from lembas.services import branding
+
+    text = branding.snapshot().text
+    return text.get(f"error_{status_code}") or text["error_500"]
 
 
 app = create_app()