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 (c73846c)