separate account settings from shop settings

- Create dedicated /admin/account page for user account management
- Move email, password, and 2FA settings from /admin/settings
- Add Account link to top of admin sidebar navigation
- Add TOTP-based two-factor authentication with NimbleTOTP
- Add TOTP verification LiveView for login flow
- Add AccountController for TOTP session management
- Remove Advanced section from settings (duplicated in dev tools)
- Remove user email from sidebar footer (replaced by Account link)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

priv/repo/migrations/20260308085927_add_totp_to_users.exs

@@ -0,0 +1,14 @@
+defmodule Berrypod.Repo.Migrations.AddTotpToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      # Encrypted TOTP secret (20 bytes base32-encoded = 32 chars, but encrypted is larger)
+      add :totp_secret_encrypted, :binary
+      # When 2FA was enabled
+      add :totp_enabled_at, :utc_datetime
+      # Backup codes (encrypted JSON array of hashed codes)
+      add :totp_backup_codes_encrypted, :binary
+    end
+  end
+end