Kacper/UI customization updates

[sc0]

Update Web UI customization examples for Nutrient SDK 1.15.1

Refreshes the two web UI-customization examples to run on the latest SDK and adopt the updated UI customization callback API, plus a round of correctness fixes to the document-editor sidebar.

Why

The examples were pinned to @nutrient-sdk/viewer@1.9.1 and used the old UI customization callback signature, where the callback received the instance directly. In current SDK versions the callback receives a getInstance getter instead, so the examples broke when run against a recent SDK. This bumps both examples to 1.15.1 and migrates them to the new API, then fixes several bugs in the document-editor sidebar that surfaced along the way.

Changes

Both examples

• Bump @nutrient-sdk/viewer 1.9.1 → 1.15.1 (lockfiles regenerated).
• Migrate custom UI slot callbacks to the new API: the callback now receives getInstance rather than instance, so the instance is resolved via const instance = getInstance() inside onMount.

web/ui-customization

• Update the commentThread slot callback to the getInstance signature.

web/ui-customization-doc-editor-sidebar

• Introduce a SIDEBAR_ID constant (customDocumentEditorSidebar) used as both the sidebar slot key and the sidebarMode view-state value, with a typed cast so a custom id type-checks against the published sidebarMode type.
• Stable page IDs: build IDs from page index + label instead of the label alone (labels aren't guaranteed unique), and use a monotonic counter for temporary (added/duplicated/imported) pages so IDs don't collide when created within the same millisecond.
• Page sizing: track each page's width/height; new and duplicated pages now match a reference page's dimensions instead of a hardcoded 300×400.
• Add page works with no selection (appends at the end) instead of requiring a selected page; the button is no longer disabled when nothing is selected.
• Move left now moves each selected page one slot at a time with individual operations, fixing incorrect results for multi-page selections under a single backward movePages.
• Import document copies the uploaded file into a fresh File before queuing the operation, so it doesn't depend on an original file handle the browser may invalidate before the operation runs.
• Button enablement: disable Remove when every page is selected (a document must keep at least one page), and disable Move Left/Move Right when the selection is already at the respective edge.

Original PR: #161
Related guides PR: https://github.com/PSPDFKit/nutrient-website/pull/2541