fix: detect Java 25 and newer Minecraft requirements - #50

Problem

The Java settings page only listed Java 8, 17, and 21. A managed Java 25 runtime downloaded by the launcher was invisible, and the version fallback did not understand year-based Minecraft versions such as 26.3.

Changes

• GetJavaStatus now builds its version list from:
  • the base majors 8, 17, and 21
  • every managed runtime under <data>/runtimes/java-*
  • every installed instance's version.json javaVersion.majorVersion
• Add java.ManagedJavaVersions().
• RequiredJavaVersion now maps year-based versions 26.1+ to Java 25, and fixes 1.21 (no patch suffix) mapping to Java 17 instead of Java 21.
• Server process Java selection now uses the shared mapping.
• Settings labels show Java 21 for 1.20.5–1.21.x and Java 25 for 26.1+.

Verification

• go test ./...
• npm run check — 0 errors
• npm run build
• Confirmed locally that ManagedJavaVersions() returns [8 17 21 25].

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