Replace browser native dialogs with Bootstrap modal system

[Copilot]

Native confirm() and alert() block JavaScript execution and provide poor UX. This PR replaces all 53 instances across the codebase with a Bootstrap-based modal system.

Changes

New dialog system (assets/js/dialogs.js):

• showConfirm() - Promise-based confirmations with danger styling for destructive actions
• showAlert() - Typed alerts (success/error/warning/info) with contextual icons
• showPrompt() - Input dialogs with keyboard support

Replaced across 28 files:

• 21 confirm() → showConfirm()
• 32 alert() → showAlert()
• Coverage: notifications, user management, eNotf protocols, settings, documents, vehicles

Key improvements:

• HTML escaping prevents XSS
• Promise resolution guards prevent race conditions
• Non-blocking execution
• Contextual titles and danger styling for delete operations

Example

Before:

if (confirm('Benachrichtigung wirklich löschen?')) {
    document.getElementById('delete-form').submit();
}

After:

showConfirm('Benachrichtigung wirklich löschen?', {
    danger: true,
    confirmText: 'Löschen',
    title: 'Benachrichtigung löschen'
}).then(result => {
    if (result) document.getElementById('delete-form').submit();
});

For inline handlers:

<form onsubmit="event.preventDefault(); showConfirm('Delete?', {danger: true}).then(r => { if(r) this.submit(); });">

Original prompt

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This section details on the original issue you should resolve

<issue_title>Standard-Browser Call-To-Action-Felder tauschen</issue_title>
<issue_description>So sachen wie return confirm('Benachrichtigung wirklich löschen?') durch ein eigenes System oder Modals oder vgl. austauschen.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Standard-Browser Call-To-Action-Felder tauschen #129

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