feat: Add --dark flag to iv for Dark Mode

[sh4shv4t]

Fixes #4710

Description

This PR implements the core Dark Mode logic for the iv viewer.

Current Implementation:

• Added a --dark command-line argument.
• Sets the Qt style to "Fusion" and applies a custom dark QPalette.
• Verified on Windows 11.

Note on Preferences:
I noticed the request in the issue to add this to the Preferences Menu for persistence. I wanted to submit the CLI implementation first to verify that the Fusion style and color palette meet the project's visual standards.

Once this styling is approved, I am happy to add the Preferences toggle and persistence logic in a follow-up commit to this PR.

Tests

• Ran iv.exe --dark test_image.jpg: Verified the UI loaded in dark mode.
• Ran iv.exe test_image.jpg: Verified the UI loaded in standard mode.

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have updated the documentation (The --help output is automatically updated).
• I am sure that this PR's changes are tested somewhere in the testsuite (Verified manually).
• I have run and passed the testsuite in CI before submitting the PR (Verified build locally).
• My code follows the prevailing code style of this project.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: sh4shv4t / name: Shashvat Singh (72ec5b3)

[sh4shv4t]

Hi @lgritz, could you please have a look at this PR once? I have added the styling via a --dark command-line argument and will add the Preferences toggle and persistence logic once the styling is approved.

[lgritz]

Sorry for the delay. I actually did test this but had mixed results.

I tried both on my personal Mac, and on my work Linux machine. In both cases, I saw minimal change, but maybe that's because I run in dark mode? Is it possible that Qt behavior is that the OS display choices explicitly choosing dark mode impose a dark palette and so this makes a visible change only if your OS and user choices are light mode? Or is it possible that it only makes a difference on certain window managers, or on MS Windows (which I couldn't test)?

Can you please post images showing the before and after appearance of iv that you expect with these changes?

It's 100% ok with me if all this effectively does is make it look dark when in light OS mode, but on effect if you've already selected dark mode and imposed a dark palette across the board. That still would be an acceptable patch.

My bigger concern, though, was that I -- totally coincidentally, I would not have consciously thought to try this -- tested it by displaying an image that happened to have an alpha channel and was transparent in some places, and this patch (versus main) definitely led to a different appearance in the "transparent" pixels. It must somehow change the background color? I would like for you to test this and track it down, we wouldn't this to inadvertently change/lighten the background pixel color for pixels with non-1.0 opacity.