feat: Add support for message attachments

- Introduced Attachment model to handle file attachments associated with messages.
- Updated ModerationController to manage message deletions and attachment cleanup.
- Enhanced ChatService to include attachments in message retrieval.
- Implemented migration for legacy single-attachment messages to the new Attachments model.
- Added unit tests for attachment handling in message formatting and parsing.
- Updated database context and migrations to support new Attachments table.

src/EchoHub.Client/UI/Helpers/DroppedFileParser.cs

@@ -0,0 +1,104 @@
+using System.Text;
+
+namespace EchoHub.Client.UI.Helpers;
+
+/// <summary>
+/// Recognizes a dragged-and-dropped file (or files) that a terminal delivers into the input as an
+/// absolute path. Terminals differ: some paste the whole path at once, others send it character by
+/// character; either way this checks whether the current input text resolves to existing file(s).
+/// </summary>
+public static class DroppedFileParser
+{
+    /// <summary>
+    /// Cheap pre-check so callers only stat the filesystem when the input plausibly holds a path:
+    /// a quoted path, a Windows drive path (<c>X:\</c>/<c>X:/</c>), a UNC path (<c>\\</c>), or a
+    /// POSIX absolute path (<c>/</c>). Normal chat text never starts this way.
+    /// </summary>
+    public static bool LooksLikePath(string text)
+    {
+        var t = text.TrimStart();
+        if (t.Length < 3)
+            return false;
+        if (t[0] is '"' or '/')
+            return true;
+        if (t.StartsWith(@"\\", StringComparison.Ordinal))
+            return true;
+        return char.IsLetter(t[0]) && t[1] == ':' && (t[2] == '\\' || t[2] == '/');
+    }
+
+    /// <summary>
+    /// Returns true when <paramref name="text"/> resolves to one or more existing files.
+    /// Handles a single path (quoted or not, possibly containing spaces) and multiple
+    /// space-separated (optionally quoted) paths. <paramref name="fileExists"/> is injectable
+    /// for testing; production passes <see cref="File.Exists"/>.
+    /// </summary>
+    public static bool TryGetFiles(string text, out List<string> files, Func<string, bool>? fileExists = null)
+    {
+        fileExists ??= File.Exists;
+        files = [];
+
+        var trimmed = text.Trim();
+        if (trimmed.Length < 3 || trimmed.Length > 4096 || trimmed.Contains('\n'))
+            return false;
+
+        // Single path, possibly quoted and/or containing spaces.
+        var unquoted = StripQuotes(trimmed);
+        if (Path.IsPathFullyQualified(unquoted) && fileExists(unquoted))
+        {
+            files.Add(unquoted);
+            return true;
+        }
+
+        // Multiple files: space-separated tokens, each optionally quoted.
+        foreach (var token in TokenizeQuoted(trimmed))
+        {
+            if (!Path.IsPathFullyQualified(token) || !fileExists(token))
+            {
+                files.Clear();
+                return false;
+            }
+            files.Add(token);
+        }
+
+        return files.Count > 0;
+    }
+
+    private static string StripQuotes(string s) =>
+        s.Length >= 2 && ((s[0] == '"' && s[^1] == '"') || (s[0] == '\'' && s[^1] == '\''))
+            ? s[1..^1]
+            : s;
+
+    private static IEnumerable<string> TokenizeQuoted(string input)
+    {
+        var current = new StringBuilder();
+        var quote = '\0';
+
+        foreach (var c in input)
+        {
+            if (quote != '\0')
+            {
+                if (c == quote) quote = '\0';
+                else current.Append(c);
+            }
+            else if (c is '"' or '\'')
+            {
+                quote = c;
+            }
+            else if (c == ' ')
+            {
+                if (current.Length > 0)
+                {
+                    yield return current.ToString();
+                    current.Clear();
+                }
+            }
+            else
+            {
+                current.Append(c);
+            }
+        }
+
+        if (current.Length > 0)
+            yield return current.ToString();
+    }
+}