launcher: v0.1 — PhotinoNET shell + multi-version orchestration

Single-download .NET 9 self-contained launcher for hosting / joining
DorkNet servers. Reads versions.json from main to discover supported
client builds, fetches the matching server binary + client patcher
from GitHub Releases per branch, manages the server subprocess +
Cloudflare quick-tunnel, and wraps it all in a PhotinoNET window.

Code layout:
- Program.cs           PhotinoNET window + message bridge bootstrap
- Backend/             AppPaths, AppState, VersionsManifest,
                       ReleaseDownloader, ServerProcess, ClientPatcher,
                       Tunnel, RecRoomPicker, JoinCode, MessageBridge
- ui/                  index.html + style.css + app.js (no build step)
- RELEASES.md          GitHub Releases artifact contract per-version
                       branches must publish to

Per-version branches need to ship two release assets per tag:
- dorknet-server-{version_key}-win-x64.zip
- dorknet-clientpatch-{version_key}.zip
(see launcher/RELEASES.md for the full naming + arg contract)

Photon AppIds, JWT secret, and the picked Rec Room path stay
per-machine; the launcher never bundles them.

launcher/Backend/JoinCode.cs

@@ -0,0 +1,67 @@
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace DorkNet.Launcher.Backend;
+
+/// <summary>Compact URL-safe encoding of the bits a joining player
+/// needs: the host's public address (typically a Cloudflare-Tunnel
+/// hostname), the Photon AppIds + region they used, the host's
+/// server name, and the version key so the joiner's launcher fetches
+/// the matching client patcher.
+///
+/// <para>Format: base64-url of a small JSON object. Roughly 200-300
+/// chars including padding — short enough to paste in chat / Discord
+/// DM without line-wrapping.</para></summary>
+public static class JoinCode
+{
+    public static string Encode(JoinPayload payload)
+    {
+        var json = JsonSerializer.SerializeToUtf8Bytes(payload);
+        return Convert.ToBase64String(json)
+            .Replace('+', '-')
+            .Replace('/', '_')
+            .TrimEnd('=');
+    }
+
+    public static JoinPayload? Decode(string code)
+    {
+        try
+        {
+            var padded = code.Replace('-', '+').Replace('_', '/');
+            switch (padded.Length % 4)
+            {
+                case 2: padded += "=="; break;
+                case 3: padded += "="; break;
+            }
+            var json = Convert.FromBase64String(padded);
+            return JsonSerializer.Deserialize<JoinPayload>(json);
+        }
+        catch { return null; }
+    }
+}
+
+public sealed class JoinPayload
+{
+    /// <summary>Public hostname the joining patcher rewrites
+    /// <c>*.rec.net</c> URIs to. Cloudflare Tunnel typically gives
+    /// <c>random-words.trycloudflare.com</c>.</summary>
+    [JsonPropertyName("host")] public string Host { get; set; } = "";
+
+    /// <summary>Version key from versions.json. Tells the joiner which
+    /// per-version branch's client patcher to fetch + install.</summary>
+    [JsonPropertyName("v")] public string VersionKey { get; set; } = "";
+
+    /// <summary>Photon Realtime AppId. The joining client patcher will
+    /// rewrite the in-binary AppId to this so the joiner reaches the
+    /// host's Photon-Cloud match instances.</summary>
+    [JsonPropertyName("pa")] public string PhotonAppId { get; set; } = "";
+
+    [JsonPropertyName("pv")] public string PhotonVoiceAppId { get; set; } = "";
+
+    [JsonPropertyName("pr")] public string PhotonRegion { get; set; } = "us";
+
+    /// <summary>Server name shown in the joiner's "you're about to
+    /// connect to ..." preview.</summary>
+    [JsonPropertyName("n")] public string Name { get; set; } = "";
+}