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]>

.env.example

@@ -1,9 +1,5 @@
 PORT=7227
 
-# Site-wide login method: "code" (passwordless, emailed code only) or
-# "password" (password, then an emailed code as a second step). Defaults to "code".
-AUTH_LOGIN_METHOD=code
-
 GOOGLE_CLIENT_ID=
 GOOGLE_CLIENT_SECRET=
 
@@ -35,6 +31,7 @@ SEED_SUPER_ADMIN_EMAIL=
 SEED_SUPER_ADMIN_USERNAME=
 SEED_SUPER_ADMIN_FIRST_NAME=
 SEED_SUPER_ADMIN_LAST_NAME=
+SEED_SUPER_ADMIN_PASSWORD=
 
 # Production: sitemap lastmod cache file on the /data volume
 PAGE_LASTMOD_FILE=