Fix restart prompt when loading text settings - #5508

• I have read the contributing guidelines, and I agree with the Code of Conduct.
• Have you checked that there aren't other open pull requests for the same changes?
• Have you tested that the committed code can be executed without errors? The affected app tests pass; broader validation limits are listed below.
• Have you confirmed that this issue is caused by UniGetUI itself, and not by the package manager or the package involved?

Opening Experimental settings loads the icon database URL into a TextboxCard. Avalonia queues TextChanged, so the handler added after assigning the initial text still receives that event. It saves the unchanged value and executes the command that displays the restart prompt.

Compare the sanitized text with the saved setting before writing or notifying. Loading an unchanged value now leaves the restart prompt hidden, while edits and clearing the field still save and notify once.

Add four headless regression cases covering empty and nonempty initial values, edits and clearing, and numeric filtering. Each test uses an isolated settings directory and headless UI session.

Validation:

• All four regression cases reproduce unwanted notifications without the guard and pass with it.
• UniGetUI.Tests: 727 passed.
• Both required whitespace and code-style checks pass.
• The broader Windows solution run passed the core and package-engine suites. The terminal-UI suite has 223 passed and one timeout in Managers_AddAndRemoveASource, waiting for the fake msstore source. The same timeout reproduces when rebuilding and running that test with the original, unmodified TextboxCard.
• The Win-x64-NativeAot publish was attempted but is blocked locally by Platform linker not found (Visual Studio Desktop Development for C++ is unavailable). Published NativeAOT runtime validation, including the other runtime profiles, remains unverified.

Closes #5507