chore(mobile): add ktlint configuration and integrate into CI #52

Problem

The mobile CI runs Android Lint but not Kotlin style linting. Without ktlint, code style diverges silently across contributors.

Acceptance criteria

• Add ktlint via the jlleitschuh/ktlint-gradle plugin (add to libs.versions.toml and build.gradle.kts)
• Add a .editorconfig with Kotlin style rules
• Wire ./gradlew ktlintCheck into the mobile-ci.yml workflow
• All existing Kotlin files pass ktlint (fix any violations)