feat(admin): PublicTicketsSettings Filament page for guest policy

[mpge]

Summary

Filament host-adapter side of the new public-ticket guest policy feature (escalated#32). Adds a new Filament admin page under Support → Public Tickets that lets admins configure the guest policy mode at runtime.

What's added

• `src/Pages/PublicTicketsSettings.php` — Filament page class mirroring the `SsoSettings` pattern already in the plugin.
• `resources/views/pages/support-public-tickets-settings.blade.php` — one-line view (standard Filament form-save template).
• `resources/lang/en/filament.php` — English strings for the page title, mode labels, section headers, and helper text.
• `src/EscalatedFilamentPlugin.php` — registers the new page in the plugin's `->pages([...])` list.

Form

• Mode dropdown (`unassigned` / `guest_user` / `prompt_signup`) with live conditional sections.
• `Shared guest user` section appears only when mode=`guest_user` with a required numeric user-id input.
• `Signup invite` section appears only when mode=`prompt_signup` with an optional URL template input (supports `{{email}}` placeholder).
• Save handler rejects unknown modes (falls back to `unassigned`) and clears fields that don't apply to the selected mode.

Persistence

Uses `Escalated\Laravel\Models\EscalatedSettings` — the same backing store as the Laravel adapter. Hosts running Filament + Laravel in the same app will see the same values whether they wrote via the Filament page or the Laravel controller.

Test plan

• Navigate to Support → Public Tickets after merge; verify all three modes save correctly and conditional fields persist only when active.
• Switch from guest_user → unassigned; verify the stored user-id is cleared.