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/utils/resolve-theme.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, test } from "bun:test";
+import resolveTheme from "./resolve-theme";
+
+describe("resolveTheme", () => {
+    test("an explicit choice wins over the device", () => {
+        expect(resolveTheme({ preference: "light", system_is_dark: true })).toBe(
+            "light",
+        );
+        expect(resolveTheme({ preference: "dark", system_is_dark: false })).toBe(
+            "dark",
+        );
+    });
+
+    test("system follows the device", () => {
+        expect(resolveTheme({ preference: "system", system_is_dark: true })).toBe(
+            "dark",
+        );
+        expect(
+            resolveTheme({ preference: "system", system_is_dark: false }),
+        ).toBe("light");
+    });
+});