Render markdown in the login disclaimer

[mvanhorn]

Summary

The login disclaimer now renders markdown. The server's BrandingOptions.loginDisclaimer is configured on the Jellyfin server and supports markdown, so a disclaimer that contains a markdown link (for example, a link to a Jellyseerr instance) renders as a tappable link instead of raw [text](url) syntax.

In Shared/Views/UserSignInView.swift, the serverDisclaimer section previously used Text(disclaimer), which renders the string as plain text. This adds a small disclaimerText(_:) helper that builds an AttributedString(markdown:) using .inlineOnlyPreservingWhitespace parsing options, and falls back to the plain string if parsing fails. Because this is a single shared view, the change covers both iOS and tvOS. The Section(L10n.disclaimer) wrapper is unchanged.

Why this matters

Closes the gap reported in #2039, where the disclaimer showed raw markdown syntax in Swiftfin. Jellyfin Web and the Jellyfin mobile app both render the disclaimer as markdown, and the reporter confirmed this with a screenshot, so this brings Swiftfin to parity with the other clients.

Testing

• A disclaimer containing a markdown link renders as a tappable link rather than raw [text](url).
• A plain-text disclaimer with no markdown renders unchanged, with whitespace preserved via .inlineOnlyPreservingWhitespace.
• An empty or nil disclaimer hides the section as before (the if let disclaimer guard is untouched).
• Malformed markdown falls back to the raw string instead of crashing or rendering nothing.
• swiftformat --lint passes on the changed file.

Fixes #2039

[LePips]

Some cleanup that can happen, or inline this if let to the section.

@ViewBuilder
private func disclaimerText(_ disclaimer: String) -> some View {
    if let attributedString = try? AttributedString(
        markdown: disclaimer,
        options: .init(interpretedSyntax: .inlineOnlyPreservingWhitespace)
    ) {
        Text(attributedString)
    } else {
        Text(disclaimer)
    }
}