feat: add Nextcloud bridge for file and calendar integration

[loxK]

Description

This PR adds an optional bridge feature that enables deep integration between RoundCube and Nextcloud services when used with the NextBridge RoundCube plugin.

Features

When enabled, users can:

• Attach files from Nextcloud - Pick files from Nextcloud storage and attach them to emails
• Save attachments to Nextcloud - Save email attachments directly to Nextcloud files
• Insert share links - Create and insert public share links into emails
• Add to Calendar - Add calendar invitations (.ics files) directly to Nextcloud Calendar

How it works

┌────────────────────────────────────────────────────────────┐
│                      Nextcloud                             │
│  ┌─────────────────────────────────────────────────────┐   │
│  │                  mail_roundcube                     │   │
│  │  ┌─────────────┐    postMessage    ┌─────────────┐  │   │
│  │  │  Vue App    │◄──────────────────│  Injected   │  │   │
│  │  │  (Bridge)   │                   │  Client     │  │   │
│  │  └──────┬──────┘                   └──────┬──────┘  │   │
│  └─────────┼────────────────────────────────┼──────────┘   │
│            │                                │              │
│     WebDAV/CalDAV                    ┌──────┴──────┐       │
│     OCS APIs                         │  RoundCube  │       │
│                                      │  (iframe)   │       │
│                                      │  NextBridge │       │
│                                      │   plugin    │       │
│                                      └─────────────┘       │
└────────────────────────────────────────────────────────────┘

1. Bridge Client Injection - When RoundCube loads, a NextcloudBridge JavaScript object is injected into the iframe
2. RoundCube Plugin - The NextBridge plugin uses this object to request file/calendar operations
3. Nextcloud Handlers - The bridge handles requests using Nextcloud's APIs (WebDAV, CalDAV, OCS Sharing)

Admin Setting

A new checkbox "Enable RoundCube bridge" in admin settings allows enabling/disabling this feature. Disabled by default.

Calendar Integration

The new CalendarController handles a specific edge case with Nextcloud's soft-delete feature:

• When an event is deleted in Nextcloud Calendar, it's soft-deleted (marked with deleted_at)
• The UID constraint remains in the database
• Re-adding an event with the same UID via CalDAV fails with 412 Precondition Failed
• The controller detects and purges orphaned UIDs before inserting new events

Related

• NextBridge - RoundCube plugin that uses this bridge
• mail_roundcube_bridge - Standalone bridge app (alternative)

Testing

1. Enable the bridge in admin settings
2. Install NextBridge plugin in RoundCube
3. Test file picking, saving, share links, and calendar integration