feat(web): add low-key monitor ID context in admin UI

[VrianCao]

What

• Show monitor IDs as secondary context across admin monitor-related surfaces (tables, test feedback, incident/maintenance monitor lists, selector options).
• In create/edit monitor form, show a subtle inline ID hint near the Name label instead of a dedicated ID block.

Why

• Monitor names are allowed to duplicate, but users can be anxious about uniqueness.
• This keeps ID visible enough to stabilize user mental model without making ID a primary admin concern.

Where

• apps/web/src/pages/AdminDashboard.tsx
• apps/web/src/components/MonitorForm.tsx
• apps/web/src/i18n/messages.ts

How to verify

• pnpm lint
• pnpm typecheck
• pnpm test
• Open Admin -> Monitors and check: name rows include subtle #ID, monitor test toasts include ID context, incident/maintenance monitor references show name + (#ID), and monitor form shows only inline ID hint near Name (not a standalone ID block).