NodeEditor: Add "Files" tab to browse node cache folder

[Copilot]

Adds a "Files" tab to the NodeEditor panel, providing an inline file browser for the node's cache folder (internalFolder), complementing the existing "Open Cache Folder" menu action.

Description

New tab visible only for computable nodes (consistent with Log/Statistics/Status tabs) that lets users browse, navigate, and open cached files without leaving the UI.

Features list

• Add "Files" tab to NodeEditor (index 4, computable nodes only)
• Navigate into subdirectories by double-clicking folders; navigate up via toolbar button
• Display file sizes (B/KB/MB/GB) alongside file names
• Open files with default OS handler or open current folder in OS file manager via toolbar button
• "No files found" placeholder when cache folder is empty or not yet computed

Implementation remarks

New NodeFileBrowser.qml component backed by Qt.labs.folderlistmodel.FolderListModel. Resets to the node's root cache folder on node change. Tab indices for Documentation (4→5) and Notes (5→6) shifted accordingly — nonComputableTabIndices, search bar enabled condition, and backdrop fallback index updated to match.

Original prompt

In the NodeEditor, add a new tab called "Files" to display the files from the node cache folder like in a file browser.

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