[Feature]: Show per-user storage usage on Dashboard instead of total DB size

[skincaz]

Is your feature request related to a problem?

Summary

Add per-user storage calculation to the Dashboard storage widget, so each
user sees only the disk usage of their own assigned mail accounts.

Problem / Motivation

In multi-user and managed service deployments, the current Dashboard shows
the total database size (e.g. 16.6 GB) to all users regardless of their role.

This causes two issues:

1. Regular users see the total storage of ALL accounts on the server,
   which is confusing and leaks information about other users' data volume.
2. For service providers billing clients based on storage consumption,
   the widget is misleading and unusable.

Describe the solution you'd like

Proposed Solution

Calculate the storage widget value based on the accounts assigned to the
currently logged-in user:

SELECT 
    SUM(octet_length(e."Subject") + octet_length(e."Body") + 
        octet_length(e."HtmlBody")) + COALESCE(SUM(a."Size"), 0)
FROM "UserMailAccounts" uma
JOIN "ArchivedEmails" e ON e."MailAccountId" = uma."MailAccountId"
LEFT JOIN "EmailAttachments" a ON a."ArchivedEmailId" = e."Id"
WHERE uma."UserId" = <current_user_id>

Alternative / Fallback

If per-user calculation is complex to implement, a simpler alternative
would be to show the total storage widget only to admin users and
hide it completely for regular users.

Additional context

Environment

• Mail-Archiver version: 2604.1
• Deployment: Docker + nginx reverse proxy
• Multi-user setup with per-account assignment via UserMailAccounts

[polskikrol]

This is a good idea, and I would also suggest adding in per-folder storage usage as well. For example, here we only say how many emails were found:

We could perhaps show a bit more information in terms of a) total DB size, b) number of attachments, etc.

I do see also what @skincaz is suggesting. I created a user account, and assigned the only email I am tinkering with so far. In the admin page here:

Having a column for storage or space or something would be useful. Then of course, that may go down the route of quotes and such, but that is another conversation.