Migrate to Nucleus 2.0 and drop platformtools/SingleInstanceManager

[kdroidFilter]

Summary

• Bump Nucleus 1.12.0 → 2.0.0-alpha-202607101252: group io.github.kdroidfilter → dev.nucleusframework (artifact names unchanged), Gradle plugin id io.github.kdroidfilter.nucleus → dev.nucleusframework.
• Migrate packages: io.github.kdroidfilter.nucleus.{darkmodedetector,graalvm,desktop.application.dsl} → dev.nucleusframework.*.
• Drop the platformtools.core dependency, replacing its OS/desktop detection with Nucleus core-runtime: getOperatingSystem()/OperatingSystem.* → Platform.Current/Platform.{Windows,MacOS,Linux,Unknown}, and detectLinuxDesktopEnvironment() → LinuxDesktopEnvironment.Current (now non-null, Unknown instead of null).
• Remove the bundled utils/SingleInstanceManager.kt; demos now use dev.nucleusframework.core.runtime.SingleInstanceManager (identical API). README updated.

Notes

• JDK toolchain stays at 17 — the Nucleus 2.0 runtimes ship Java 11 bytecode, and no IntelliJ-snapshots repo is required (no transitive Jewel deps).
• TrayAppDemo keeps application { }: TrayApp/Tray require Compose's ApplicationScope (for DialogWindow), which nucleusApplication does not expose publicly.

Test plan

• ./gradlew :compileKotlinJvm :demo:compileKotlinJvm
• Run the demo and verify tray + single-instance behavior on each OS