feat: Add custom folder mounts configuration

[W1talka]

Summary

Add the ability for users to mount custom folders from the Linux host
filesystem into the Windows VM. Mounted folders appear in Windows at
\\host.lan\Data\<name>.

Features:

• Add/remove custom folder mounts via the Configuration panel
• Browse for folders using native file picker
• Enable/disable individual mounts without removing them
• Path validation (checks directory exists and is accessible)
• Share name validation (alphanumeric, underscores, hyphens only)

Changes

• CustomVolumeMounts.vue - New component for managing mounts
• volumes.ts - Validation helpers and compose config utilities
• types.ts - New CustomVolumeMount type
• Config.vue - Integration into settings page
• config.ts / winboat.ts - Backend integration

Test Plan

• Add a new mount via folder browser
• Add a mount by typing path manually
• Verify validation errors appear for invalid paths
• Toggle mount on/off
• Remove a mount
• Restart WinBoat and verify mounts persist
• Access mounted folder from Windows at \\host.lan\Data\<name>

Closes #343
Closes #44