Add Filament admin interface to manage and preview default mail notifications (registration, password reset, etc.)

[Devsome]

We need a dedicated Filament resource/page that allows administrators to view, edit, and preview all standard mail notifications used by the application — specifically the emails sent for confirm registration, forgot password, reset password, and similar authentication‑related messages.

Currently these emails use default Laravel notification templates that are difficult to customise without modifying vendor files or overriding views manually. The goal is to provide a user‑friendly admin interface inside Filament where:

• All default mail templates are listed (name, subject, last updated).
• Each template’s content can be edited (subject line, body – plain text and/or HTML).
• A live preview or render of the email is available before saving changes.
• Customised templates are stored in the database and automatically used by the application instead of the defaults.

This eliminates the need for manual code deployments to adjust email wording and allows server operators to quickly adapt messages to their community without developer assistance.

Acceptance Criteria:

• A new Filament resource (e.g. MailTemplateResource) or custom page is accessible in the admin panel.
• Default templates for at least: email verification (confirm registration), password reset, and forgot password are displayed and editable.
• Editing provides fields for subject, body (Markdown/HTML), and optional variables (e.g. :name, :reset_url).
• A preview panel shows a rendered version of the email with sample data.
• Changes are saved to the database and the application sends the customised versions instead of the original Laravel ones.
• Sane fallback to default templates if no customisation exists.