feat: Add dark mode support

[thiagokroger]

Implements dark mode theming across the application:

Important to know:

• The whole theme selector is only available through an Env var called VITE_ENABLE_THEMES
• Light mode is still the default, once we are confident we can let system be the default.

Features added

• Add theme toggle with system preference detection
• Update UI components to support dark/light themes
• Ensure proper contrast and accessibility in dark mode
• Default to light theme with user preference persistence

Closes #1128

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Note

Adds first-class dark mode across the app with theme persistence and system detection.

• New ThemeProvider/useTheme and ThemeSelector (gated by VITE_ENABLE_THEMES via isThemeConfigEnabled) replace useDarkMode; App wraps content and Toaster/dialogs adapt to effectiveTheme
• Global CSS updates: .dark variables, scrollbar/JSON viewer dark styles, and Tailwind @custom-variant dark (&:is(.dark *))
• Broad UI adjustments to support dark classes (inputs, buttons, selects, labels, text, panels, modals, headers, auth pages, settings, canvas, logs)
• Standardized integration icons via APP_LOGO_MAP and DARK_ICONS_NEEDING_INVERT; components/mappers now pass appName instead of iconSrc where applicable
• New isThemeConfigEnabled env util; numerous pages conditionally render Appearance section with ThemeSelector
• Minor dependency bump: html-to-image 1.11.11 → 1.11.13

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