feat(app): wire Check for Updates to releases page, add DMG packaging script

About screen's "Check for Updates..." was a no-op placeholder - opens
the Gitea releases page now (no Sparkle-style in-app updater yet).

scripts/package-dmg.sh packages an Xcode-exported .app into a
distributable DMG with an Applications symlink, versioned from the
app's own Info.plist.

Outpost/Features/About/AboutView.swift

@@ -1,8 +1,10 @@
 #if os(macOS)
+import AppKit
 import SwiftUI
 
 struct AboutView: View {
     private let repositoryURL = URL(string: "https://git.psmattas.com/psmattas/Outpost")!
+    private let releasesURL = URL(string: "https://git.psmattas.com/psmattas/Outpost/releases")!
 
     private var appName: String {
         Bundle.main.object(forInfoDictionaryKey: "CFBundleName") as? String ?? "Outpost"
@@ -71,7 +73,10 @@ struct AboutView: View {
         .frame(width: 320)
     }
 
-    // Update checking isn't wired to a distribution channel yet — button is a placeholder.
-    private func checkForUpdates() {}
+    // No Sparkle-style in-app updater yet — this just opens the releases page
+    // on the self-hosted Gitea instance so the user can check/download manually.
+    private func checkForUpdates() {
+        NSWorkspace.shared.open(releasesURL)
+    }
 }
 #endif

scripts/package-dmg.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+# Packages an already-exported Outpost.app into a distributable DMG with an
+# Applications-folder symlink for drag-to-install.
+#
+# Usage: scripts/package-dmg.sh /path/to/Outpost.app [output-dir]
+#
+# Produces: <output-dir>/Outpost-<version>.dmg
+# Version/build number are read directly from the app's own Info.plist, so
+# this always matches whatever MARKETING_VERSION was set to at archive time.
+
+set -euo pipefail
+
+APP_PATH="${1:?Usage: package-dmg.sh /path/to/Outpost.app [output-dir]}"
+OUT_DIR="${2:-$(pwd)/dist}"
+
+if [ ! -d "$APP_PATH" ]; then
+    echo "error: '$APP_PATH' not found or not a directory" >&2
+    exit 1
+fi
+
+APP_NAME="$(basename "$APP_PATH" .app)"
+VERSION="$(defaults read "$APP_PATH/Contents/Info" CFBundleShortVersionString)"
+BUILD="$(defaults read "$APP_PATH/Contents/Info" CFBundleVersion)"
+
+mkdir -p "$OUT_DIR"
+
+STAGING_DIR="$(mktemp -d)"
+trap 'rm -rf "$STAGING_DIR"' EXIT
+
+echo "Staging '$APP_NAME.app' ($VERSION build $BUILD)..."
+cp -R "$APP_PATH" "$STAGING_DIR/"
+ln -s /Applications "$STAGING_DIR/Applications"
+
+DMG_NAME="${APP_NAME}-${VERSION}.dmg"
+DMG_PATH="$OUT_DIR/$DMG_NAME"
+rm -f "$DMG_PATH"
+
+echo "Building $DMG_NAME..."
+hdiutil create \
+    -volname "$APP_NAME $VERSION" \
+    -srcfolder "$STAGING_DIR" \
+    -ov \
+    -format UDZO \
+    "$DMG_PATH"
+
+echo "Done: $DMG_PATH"