init: java devshell

.envrc

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+export DIRENV_HIDE_ENV_DIFF=1
+use flake

README.md

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+# Mental Wellness Tracker (Java devshell)
+
+Enter the shell with:
+
+```
+nix develop
+```
+
+Build examples:
+
+### Maven
+```
+mvn package
+```
+
+### Gradle
+```
+gradle build
+```

flake.nix

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+{
+  description = "Dev shell: Java (JDK 21, Maven, Gradle)";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in {
+        devShells.default = pkgs.mkShell {
+          name = "java-devshell";
+
+          packages = with pkgs; [
+            jdk21
+            maven
+            gradle
+            jdt-language-server
+          ];
+
+          shellHook = ''
+            export JAVA_HOME=${pkgs.jdk21}
+            echo "☕ Java devshell ready — JDK 21, Maven, Gradle"
+          '';
+        };
+      });
+}