Phase 1.0b: Convert all Gradle build scripts to Kotlin DSL

[sjaramillo10]

Linked issue

Closes #3.

What changed

• Every .gradle file in the repo became .gradle.kts: settings.gradle, root build.gradle, detekt/detekt.gradle, and all 14 module build files. Pure syntax conversion on top of the 1.0a version catalog — no dependency or version changes.
• settings.gradle.kts adds a pluginManagement block with gradlePluginPortal(), google(), and mavenCentral(). Required because Paparazzi's transitive com.android.tools:sdk-common only resolves from Google Maven.
• detekt/detekt.gradle.kts removed; detekt config folded into root build.gradle.kts. Script plugins loaded via apply(from = ...) run with their own compilation classpath and can't see plugin extension types (DetektExtension). Inlining into the allprojects {} block preserves identical behavior with cleaner KTS semantics.
• Plugin application convention captured during conversion:
  • Plugins on the buildscript classpath (AGP, Kotlin, Karumi shot, Vanniktech maven-publish) use plain id("...") in module scripts with no version.
  • Plugins resolved through pluginManagement (KSP, compose-compiler, paparazzi, detekt) use alias(libs.plugins.*).
  • Mixing the two for the same plugin produces "plugin already on classpath with unknown version" errors. Documented in CLAUDE.md.
• One deviation from strict syntax-only conversion: the afterEvaluate { ... } block in showkase-screenshot-testing-paparazzi-sample/build.gradle was a workaround for KSP issue Setup for testing the annotation processor airbnb/Showkase#37 (fixed in KSP 1.0.7, June 2022). KSP 2.x — what we run — registers generated Kotlin sources automatically, so the block was already a no-op. Dropped during conversion with an inline comment explaining the rationale and how to restore if a regression surfaces.
• CLAUDE.md updated: notes the Kotlin DSL state, plugin convention, pluginManagement repos, and prunes the now-shipped catalog/KTS entries from the "Keep this file in sync" examples.

How to validate locally

./gradlew check assembleDebug :showkase-processor:test

Ran locally: green in 3m30s, 762 actionable tasks executed. CI runs the same.

CI status

ci.yml will run check assembleDebug :showkase-processor:test on JDK 17. Expecting green.

Out-of-scope follow-ups

• Phase 1.1 (1.1 Drop KAPT entirely (KSP-only path) #5): drop the useKsp toggle and KAPT branches everywhere. Plugin alias kotlin-kapt becomes unused and can be removed from the catalog.
• Phase 1.2 (1.2 Bump toolchain: AGP 9, Kotlin 2.3, KSP2, Compose 1.11, Gradle 9.1+, Detekt #6): bump AGP / Kotlin / KSP / Compose / Gradle; surfaces a number of AGP DSL deprecation warnings during this run (packagingOptions → packaging, library targetSdk, etc.) that should be addressed at the toolchain bump.
• Phase 1.2 follow-up: re-evaluate whether vanniktech maven-publish can move off the buildscript classpath entirely and use plugin-portal resolution (one of the few duplications between buildscript classpath and root plugins block).