Transmission #046: Improve the Gradle version catalog

Cleaned up the server’s Gradle dependency management by moving toward (and documenting) a standard version catalog flow.

• Captured PDD artifacts for the migration (planning, detailed design, and an implementation plan) under .agents/planning/2026-04-23-gradle-version-catalog/.
• Migrated the server build to use server/gradle/libs.versions.toml and libs.* aliases instead of the old custom dependencies.gradle maps.
• Did a small follow-up tidy: alphabetized the catalog’s [versions] entries so it’s easier to scan and maintain.
• Followed up by grouping the server’s build.gradle dependencies and sorting entries alphabetically within each group to keep future dependency diffs clean.