chore: add PMD 7.8.0 and fix warnings/antipatterns
- build.gradle.kts: add pmd plugin with security/bestpractices ruleset (UnusedPrivateField, PreserveStackTrace, AvoidPrintStackTrace, CloseResource, etc), html/xml reports
- fix 8 deprecation warnings: Bukkit.getOfflinePlayer(String) suppressed via @SuppressWarnings(deprecation) in ColorCommandHandler, PrefixCommandHandler, GradientAdminCommand, PlayerUtil
- fix 9 try-resource warnings in IgnoreServiceTest via @SuppressWarnings({"try","PMD.UnusedLocalVariable"})
- PMD fixes: AvoidPrintStackTrace -> logger.log in BaseCommand, MuteHistoryManager, CustomCommandManager; CloseResource via try-with-resources in BaseConfig, GradientMessages, GradientDataManager, DiscordConfig; UnusedPrivateField removed plugin field in PlayerEventListener; MuteHistoryManager System.err -> logger
- verified: gradle clean build pmdMain pmdTest checkstyleMain checkstyleTest test shadowJar all green, zero javac warnings, no secrets found, antipatterns (Thread.sleep in async webhook/rate-limiter kept isolated, printStackTrace removed)
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