Reset project: toolchain upgrade, design-system fixes, tooling

[tamzi]

Summary

Brings the update-project-reset branch up to date: a full toolchain and dependency
upgrade, the low-risk design-system fixes from the code audit, repaired build/CI tooling,
and the reconciled findings.md / backlog.md docs.

Changes

Toolchain and dependencies

• Gradle wrapper to 9.6.0, with the foojay resolver and a pinned daemon JVM (21).
• Version catalog moved to current latest stable across the board (AGP 9.2.1, Kotlin 2.4.0,
  Compose BOM 2026.06.00, lint 32.2.1, JUnit BOM 6.1.0, AndroidX stack). Verified against the
  live Maven repositories.
• Android modules raised to Java 21, consistent with chailinter.
• Unit tests migrated to JUnit 5 (Jupiter) with useJUnitPlatform(); the old JUnit 4 entry
  and unused plugin aliases removed.

Design system

• ChaiColors wired into MaterialTheme; buttons read the palette directly.
• CPrimaryButton gets a branded container; the dead Space* no-op statements are real spacers.
• Buttons demo screen now renders the actual components.
• ChaiTheme draws edge-to-edge instead of the deprecated window.statusBarColor.
• Typography switched to staticCompositionLocalOf, matching the colors token.

Tooling and docs

• Repaired setup.sh, removed the broken spotless init-script references, pointed the todo
  workflow at develop, dropped the obsolete enableJetifier property.
• findings.md and backlog.md pruned to reflect what is resolved versus still open.

How to test

./gradlew build
./gradlew test
./gradlew lint detekt

CI runs the same on this PR.

Issue

No linked issue — this is project-reset / maintenance work.