Build offline OpenPrestige release bundles

Rename the injector and package it with an embedded payload for Windows, Linux, and macOS x64/ARM64. Add a core-only ImGui native build, multi-platform CI and release packaging, and shared per-user storage with legacy config migration.

src/main/java/dev/zprestige/prestige/client/AppPaths.java

@@ -0,0 +1,42 @@
+package dev.zprestige.prestige.client;
+
+import java.nio.file.Path;
+
+public final class AppPaths {
+    private static final Path ROOT = root();
+
+    private AppPaths() {}
+
+    public static Path configs() {
+        return ROOT.resolve("configs");
+    }
+
+    public static Path runtime() {
+        return ROOT.resolve("runtime");
+    }
+
+    private static Path root() {
+        String os = System.getProperty("os.name", "");
+        Path base;
+        if (os.startsWith("Windows")) {
+            base = env("APPDATA");
+            Path home = env("USERPROFILE");
+            if (base == null && home != null) base = home.resolve("AppData").resolve("Roaming");
+        } else {
+            Path home = env("HOME");
+            if (os.startsWith("Mac")) {
+                base = home == null ? null : home.resolve("Library").resolve("Application Support");
+            } else {
+                base = env("XDG_DATA_HOME");
+                if (base == null || !base.isAbsolute()) base = home == null ? null : home.resolve(".local").resolve("share");
+            }
+        }
+        if (base == null) base = Path.of(System.getProperty("java.io.tmpdir"));
+        return base.resolve(".openprestige");
+    }
+
+    private static Path env(String name) {
+        String value = System.getenv(name);
+        return value == null || value.isEmpty() ? null : Path.of(value);
+    }
+}