Make Cobble a local install: setup flow, bots-first admin, settings

Local, password-only auth:
- No public pages; everything outside /admin and /auth redirects in
- First run (no users) sends every page to /auth/signup, which creates
  the super admin (user #1) once and then locks
- Login is email/username + password; removed the emailed-code flow,
  /auth/sso and email password reset
- Google login kept for existing accounts only, never creates them
- seed-super-admin now requires SEED_SUPER_ADMIN_PASSWORD

Bots-first admin:
- /admin and closed sections (people, logs, backups) redirect to
  /admin/bots; only bots and settings are live (AdminSections)
- Removed the old admin layout; every /admin page uses the bots rail
- Coordinator bot created at signup/seed and by /admin/bots
  (ensureCoordinatorBot), which redirects to its chat
- bots.system_prompt (markdown) with a default coordinator prompt
- seed-bots reuses the existing coordinator

Settings:
- Sub pages general (default), profile, change-password, listed in the
  rail with a back link; unknown settings paths redirect to general
- Redesigned settings UI: pane header, grouped rows, sticky save bar
- General: System/Light/Dark theme, saved per device

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/pages/admin/settings/(partials)/settings-pane-header.tsx

@@ -0,0 +1,28 @@
+import Row from "@moduletrace/twui/layout/Row";
+import Stack from "@moduletrace/twui/layout/Stack";
+import H1 from "@moduletrace/twui/layout/H1";
+import Span from "@moduletrace/twui/layout/Span";
+
+type Props = {
+    title: string;
+    description: string;
+};
+
+/**
+ * Same 64px header as a conversation, so moving between bots and settings
+ * doesn't shift the page.
+ */
+export default function SettingsPaneHeader({ title, description }: Props) {
+    return (
+        <Row className="h-16 shrink-0 gap-3! flex-nowrap items-center px-6 border-0 border-b border-solid border-hairline">
+            <Stack className="min-w-0 gap-0!">
+                <H1 className="text-lg! leading-6! tracking-normal! font-bold text-ink truncate">
+                    {title}
+                </H1>
+                <Span className="text-[13px] leading-[18px] text-ink-secondary truncate">
+                    {description}
+                </Span>
+            </Stack>
+        </Row>
+    );
+}