Add "Remember my choice" option for protected view notice

[Copilot]

PR Checklist

• Functionality has been tested, no obvious bugs
• Code style follows project conventions
• Documentation/comments updated (if applicable)

Brief Description of Changes

The OfficeViewer plugin now prompts users once for their preference when handling protected files, then remembers their choice.

Implementation:

• Added ProtectedViewDialog with "Remember my choice" checkbox
• Store preference as nullable bool in AlwaysUnblockProtectedView setting
• Skip dialog when preference exists, apply saved choice directly
• Support remembering both "Yes" (auto-unblock) and "No" (remain blocked) choices

Behavior:

First encounter → Show dialog with checkbox
User checks "Remember" + "Yes" → All future protected files auto-unblock
User checks "Remember" + "No" → All future protected files skip unblock
User unchecked → Dialog appears for each file

Related Issue (if any)

Discussion #1831

Additional Notes

Uses nullable bool (bool?) to distinguish "no preference set" from "preference set to false".

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Prevent the protected view notice!</issue_title>
<issue_description>

Discussed in #1831

^{Originally posted by cruisermaxx December 7, 2025}
When preview some .docx file, there is a messagebox about protected view.
Is it possible to remember the choice Yes or No? Don't popup very time.

<agent_instructions>add a bool option to if remember the Yes choice</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Prevent the protected view notice! #1832

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.