feat(dev): add scripts/dev-seed to populate sample test data

New scripts/dev-seed.sh (+ .ps1), run after dev-server-init: seeds the
test server with Users (alice, bob, carol — spanning User/Admin roles,
group membership, an alias), Groups (engineering, marketing), Mailing
Lists (newsletter, support), and custom Roles (Support Agent,
Read-only Auditor), so each admin panel screen has real data to test
against instead of an empty list.

Idempotent (skips if 'alice' already exists), documented as workflow
step 3 in DEVELOPMENT.md (renumbering the steps after it) and listed
in AGENTS.md. Verified end-to-end with both scripts against a fresh
server, including idempotency and the resulting UI (Accounts, Groups,
Mailing Lists, Roles all populated as expected).

AGENTS.md

@@ -10,6 +10,7 @@ Community fork of [stalwartlabs/webui](https://github.com/stalwartlabs/webui), a
 
 - `npm run dev:server` - Start a local disposable Stalwart test server (Docker); `npm run dev:server:down` to stop it
 - `bash scripts/dev-server-init.sh` (or `pwsh ./scripts/dev-server-init.ps1`) - One-time setup of that server (bootstrap, dev admin account, 3h default token lifetime); idempotent
+- `bash scripts/dev-seed.sh` (or `pwsh ./scripts/dev-seed.ps1`) - Optional: seed sample Users/Groups/Mailing Lists/Roles for testing; idempotent
 - `bash scripts/dev-token.sh [duration_seconds]` (or `pwsh ./scripts/dev-token.ps1 [-DurationSeconds N]`) - Get a dev access token from that server, 3h by default
 - `npm run dev` - Dev server (proxies `/api` and `/jmap` to the test server)
 - `npm run typecheck` - `tsc --noEmit`