fix: use portable grep -oE for Java version detection on macOS

[WeToolX]

Fixes #27.

Problem

check-deps.sh fails on macOS because it uses the GNU-only grep -oP flag, which is not supported by BSD grep:

grep: invalid option -- P

This fallback runs when the primary sed extraction returns empty, which happens for OpenJDK versions that report a plain major version such as openjdk version "25".

Change

Replace the GNU-only regex with a portable one:

-    java_version=$(echo "$java_version_output" | grep -oP '\d+' | head -1)
+    java_version=$(echo "$java_version_output" | grep -oE '[0-9]+' | head -1)

Verification

On macOS (Darwin), after the change:

[OK] Java 25 detected
[OK] jadx 1.5.5 detected
[OK] vineflower CLI detected
[OK] dex2jar detected
[OK] apktool detected (optional)
[OK] adb detected (optional)

All dependencies are installed. Ready to decompile.

One-line change, no behavioral impact on Linux/GNU grep.