Add options for local HTML view and a faster way to add storages to parts

[kernchen-brc]

Summary

This PR adds an "Add stock" button to the part stock information page.

Motivation

Previously, adding new stock entries required navigating away from the part stock
information view. This change improves usability by allowing users to create stock
entries directly from the relevant page.

Changes

• Added an "Add stock" button to the part stock info page

Result

• Fewer clicks to add stock

[kernchen-brc]

The new commit adds a checkbox that allows disabling the warning banner for user-uploaded HTML attachments. The banner remains enabled by default.

[kernchen-brc]

Last commit in this pull request.

Add a new system setting "Allow HTML attachments to use browser storage"
(disabled by default) that controls whether sandboxed HTML attachments
can access the browser's localStorage/sessionStorage via allow-same-origin.

When disabled (default), attachments run in an opaque origin as before,
preventing any persistent storage. When enabled, data survives page
reloads but grants the attachment access to Part-DB's browser origin.