Custom emoji, downvotes, blurhash and focal points on the cards

What other platforms send now shows as they meant it:
- custom emoji (a post's or account's :shortcode:) as their pictures in
  posts, names, content warnings, bios and fields: only the shortcodes the
  server listed, only http(s) pictures, never inside a tag, the shortcode
  kept as alt text;
- a threadiverse post's downvotes next to its favourites;
- blurhash placeholders until a picture loads (wwwroot/js/blurhash.js, a
  small decoder that watches the page), and a picture's focal point;
- an event's online link, participants and capacity, and whether it is
  tentative or cancelled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Helpers/CustomEmojis.cs

@@ -0,0 +1,47 @@
+using System.Net;
+using System.Text.RegularExpressions;
+
+using decePubClient.Models.Mastodon;
+
+using Microsoft.AspNetCore.Components;
+
+namespace decePubClient.Helpers
+{
+	/// <summary>
+	/// A post's or an account's custom emoji (`:shortcode:` among its <c>emojis</c>) shown as their pictures. The shortcode
+	/// stays as the picture's alt text, so it is what a screen reader reads and what is copied. Only shortcodes the
+	/// server listed for that post or account are replaced, and only with an http(s) picture.
+	/// </summary>
+	public static partial class CustomEmojis
+	{
+		[GeneratedRegex(@":([A-Za-z0-9_+-]{1,64}):")]
+		private static partial Regex Shortcode();
+
+		[GeneratedRegex(@"(<[^>]*>)")]
+		private static partial Regex Tag();
+
+		/// <summary>Plain text (a name, a content warning): escaped, then its shortcodes as pictures.</summary>
+		public static MarkupString Text(string text, IReadOnlyCollection<CustomEmoji> emojis) =>
+			new(Replace(WebUtility.HtmlEncode(text ?? string.Empty), emojis));
+
+		/// <summary>HTML the server sanitized: the shortcodes in its text, never inside a tag.</summary>
+		public static string Html(string html, IReadOnlyCollection<CustomEmoji> emojis) =>
+			emojis.Count == 0 || string.IsNullOrEmpty(html)
+				? html
+				: string.Concat(Tag().Split(html).Select(part => part.StartsWith('<') ? part : Replace(part, emojis)));
+
+		static string Replace(string text, IReadOnlyCollection<CustomEmoji> emojis)
+		{
+			if (emojis.Count == 0 || !text.Contains(':'))
+				return text;
+			var known = new Dictionary<string, string>(StringComparer.Ordinal);
+			foreach (var emoji in emojis)
+				if (!string.IsNullOrEmpty(emoji.Shortcode) && Uri.TryCreate(emoji.Url, UriKind.Absolute, out var url)
+					&& url.Scheme is "https" or "http")
+					known.TryAdd(emoji.Shortcode, url.AbsoluteUri);
+			return Shortcode().Replace(text, match => known.TryGetValue(match.Groups[1].Value, out var url)
+				? $"<img class=\"custom-emoji\" src=\"{WebUtility.HtmlEncode(url)}\" alt=\"{match.Value}\" title=\"{match.Value}\" draggable=\"false\" />"
+				: match.Value);
+		}
+	}
+}