feat(webapp): align standalone UI with DVLS theme

[Krista House (kristahouse)]

Aligns the standalone Gateway webapp styling with the DVLS theme, adding light and dark theme support and refreshing the main UI surfaces users interact with.

This updates the sidebar, login page, session backgrounds, dropdowns, reconnect form layout, and toast spacing so the standalone Gateway webapp feels consistent with the DVLS experience.

Issue: N/A

[Krista House (kristahouse)]

Implementation notes:

• Added a ThemeService plus early document theme class setup so PrimeNG and Gateway styles can share .dark-theme behavior without flicker.
• Switched PrimeNG to the DVL preset and copied the DVLS/demo token stack for standalone Gateway surfaces.
• Kept the sidebar in a fixed dark token scope to match DVLS while allowing the main workspace to switch themes.
• Pinned @devolutions/iron-remote-desktop-vnc to 0.7.0 because 0.7.1 caused ARD sessions to connect but render transparent in Gateway; 0.7.0 matches DVLS and fixes the rendering path.
• Scoped the PR to theme/UI foundation only; Active Directory functionality and AD-specific package wiring are intentionally excluded for the stacked follow-up PR.

Validation:

• pnpm --filter gateway-ui lint
• pnpm --filter gateway-ui build
• Manual smoke test of light/dark theme, sidebar, login, dropdowns, reconnect form layout, toast spacing, and ARD rendering

[github-actions]

Let maintainers know that an action is required on their side

• Add the label release-required Please cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module) when you request a maintainer to cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module)

• Add the label release-blocker Follow-up is required before cutting a new release if a follow-up is required before cutting a new release

• Add the label publish-required Please publish libraries (`Devolutions.Gateway.Utils`, OpenAPI clients, etc) when you request a maintainer to publish libraries (Devolutions.Gateway.Utils, OpenAPI clients, etc.)

• Add the label publish-blocker Follow-up is required before publishing libraries if a follow-up is required before publishing libraries