1.0a Migrate to Gradle version catalog (libs.versions.toml) #2

Goal

Centralize dependency coordinates in gradle/libs.versions.toml while keeping all build files in Groovy DSL. This isolates dep-management changes from the KTS conversion that follows in 1.0b.

Scope

• Create gradle/libs.versions.toml.
• Translate the ext.versions and ext.deps maps currently in root build.gradle into the catalog's [versions], [libraries], and [plugins] sections.
• Refactor every module's build.gradle to consume libs.* accessors. Build files stay Groovy.
• Delete the ext.versions / ext.deps blocks from the root build.gradle once nothing references them.
• Detekt plugin coordinates also move into the catalog.

CI gate

• ./gradlew check assembleDebug :showkase-processor:test green.
• No runtime/behavioral changes; this is a pure refactor.

Out of scope

• KTS conversion (1.0b).
• Dependency version bumps (1.2). Match current versions exactly.
• Adding new dependencies.

Notes for execution

• Version bundles can be useful for the deps.compose.* cluster — group it as compose bundle.
• Plugin aliases: kotlin-android, kotlin-jvm, android-library, android-application, compose-compiler, ksp, detekt, maven-publish, shot.
• Keep KSP_VERSION / KOTLIN semantics by binding to single version refs in [versions].