Preserve original file extension in WebDAV temporary files

[jim-kirisame]

📝 Description

Some WebDAV providers do not allow changing file extensions. This change modifies the temporary file naming to insert the .tmp suffix before the extension instead of after it, e.g. file.kdbx becomes file.tmp.abc12345.kdbx instead of file.kdbx.tmp.abc12345.

🔗 Related Issue

None.

🛠️ Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

✅ Checklist

Before submitting this PR, please confirm the following:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own (or any AI generated) code.
• I have added/updated tests that prove my fix is effective or that my feature works.
• All new and existing tests passed locally.
• I have commented my code, particularly in hard-to-understand areas.
• I have built the app locally, verified that it builds and tested the changes thoroughly.

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📸 Screenshots/Videos (if applicable)