ui: replace native window.confirm with custom theme-aware modal

[JonanOribe]

The goal of this branch is to unify the styles of the application's modal windows, which currently use the browser's default styling.

The first iteration aimed to:

• Replace browser-default confirm dialogs with a custom modal to
  ensure consistent look and feel across different browsers (Chrome,
  Firefox, Safari, etc.).
• Native alert/confirm dialogs break the platform's visual identity,
  cannot be styled, and fail to adapt to the user's selected
  theme (light/night mode).
• Unifying cross-browser UI components prevents jarring visual
  discrepancies and provides a cohesive, professional user experience
  aligned with the tactical dashboard design.

Before:

Now:

The next step in these iterations will involve identifying all the application's modals to achieve complete unification.

[smittix]

Thanks @JonanOribe, this looks much better than the browser dialog, and it's good to have it follow the theme. A few things before this becomes the shared confirm for the rest of the app (you mentioned that as the next step):

1. Make it reusable: showCustomConfirm(title, message, onConfirm) reads as generic, but the button text is fixed to "Yes, Stop All". Could the confirm label be a parameter (default "Confirm")?
2. Use the theme variables throughout: the light and dark colour sets are almost identical (they already use the CSS variables), so one set would do. The title colours are fixed hex values (#dc2626 / #f87171), and those could be var(--accent-red). The system-ui font could be the app's own font variables so the pop-up matches the rest of the UI.
3. Keyboard and dismissal: Escape and clicking the dark backdrop should cancel. Focus should move into the pop-up when it opens (ideally to Cancel, since this is a destructive action) and go back to the Kill All button when it closes.
4. Small one: the closing } of showCustomConfirm has lost its indentation.

None of these block this first step on its own, so it's fine to cover them in your follow-up PR when the other dialogs move across, as long as 1 lands before anything else uses it. Thanks again!

[JonanOribe]

Already done @smittix