fix(plugin): allow applying org.jetbrains.compose alongside Nucleus

[kdroidFilter]

Fixes #284.

Summary

Nucleus forks the Compose Desktop packaging and duplicated two of the Compose plugin's registrations, causing order-dependent conflicts when both org.jetbrains.compose and the Nucleus plugin are applied to the same module (commonly surfaced during IntelliJ Gradle sync).

Two distinct bugs, both fixed:

1. Duplicate jetbrainsCompose extension (nucleus-before-compose order) — the Groovy-DSL setup was gated behind !project.buildFile.endsWith(".gradle.kts"). File.endsWith(String) matches by path components, not by string suffix, so File("build.gradle.kts").endsWith(".gradle.kts") is always false → the Groovy setup ran for Kotlin DSL projects too and registered a jetbrainsCompose repositories extension, which the Compose plugin then fails to add.

2. Task-name collision (compose-before-nucleus order + mainClass set) — propagateMainClassToComposeApplication called the lazy getApplication getter, which initializes compose.desktop.application as a side effect, making the Compose plugin register its own packaging tasks that collide with Nucleus's identically-named ones (unpackDefaultComposeDesktopJvmApplicationResources, createDistributable, packageDmg, …).

Changes

• Detect Kotlin DSL via buildFile.name.endsWith(".gradle.kts") so the Groovy setup only runs for Groovy build files.
• Forward mainClass only when Compose's application is already initialized (explicit block or Hot Reload plugin), read via the internal _isJvmApplicationInitialized flag without triggering initialization. Nucleus therefore never causes Compose to register packaging tasks — only Nucleus's forked, modified tasks exist.
• Fail fast with an actionable message when both nucleus.application { } and compose.desktop.application { } are configured (the Compose plugin can stay applied for Hot Reload / IDE integration).

Test plan

• Reproduced [Enhancement] Decouple Nucleus Plugin from Compose Desktop #284 on a vanilla CMP template + the reporter's repo (published 2.0.1)
• nucleus-before-compose order: no longer crashes on jetbrainsCompose
• compose-before-nucleus order (no compose block): :desktopApp:run succeeds
• Both blocks configured: clear actionable error instead of cryptic Gradle failure
• Packaging task classes are 100% dev.nucleusframework.* even with org.jetbrains.compose applied (no Compose Desktop tasks used)
• Compose Hot Reload (runHot) still starts
• :plugin:test, ktlint and detekt pass