feat(java): Download JDK, and one-click "Download JDK N" where a JDK is missing (#61)

Root cause: the IDE only detected installed JDKs and had no way to get one, so any launch needing a newer JDK than the machine has (latest jdtls needs JDK 25) dead-ended in an error.

Fix: IDEA-style Download JDK (Eclipse Temurin via the Adoptium API, sha256-verified, reusing ide-tools' download/verify code; Windows zip, Linux tar.gz, macOS Contents/Home) into <app-data>/tools/jdks, registered with JDK discovery and rescanned after each download. "Download JDK…" in the Project SDK and Language Server JDK pickers; a one-click `java.downloadJdk` "Download JDK N" on "needs JDK N" errors, which also repoints a too-old language server JDK and restarts jdtls.

Siblings: language server status popup, jdtls launch notification, run console classpath error, debug launch, run configurations without a JDK ≥ the module's Java version, Maven/Gradle sync failures, single-file Java run without `java` on PATH.

Verified in the app on macOS: vendor/version/location queries, the Download JDK dialog from the Language Server JDK picker, a real Temurin 25.0.5 download that runs. Not verified on Windows/Linux.

Fixes #49

https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src-tauri/src/commands/java.rs

@@ -48,6 +48,7 @@ pub async fn java_resolve_launch(
     let java = project_jdk.map(|j| ide_sdk::java::java_executable(&j.home)).or(from_jdtls).or_else(|| {
         let release = ide_sdk::java::project_java_version(&module);
         let jdks = ide_sdk::java::installed_jdks();
+        info.hints.extend(ide_sdk::java::missing_jdk_message(&jdks, release));
         ide_sdk::java::pick_jdk(&jdks, release).map(|j| ide_sdk::java::java_executable(&j.home))
     });
     info.java_exec = Some(java.map(|p| p.to_string_lossy().into_owned()).unwrap_or_else(|| "java".to_owned()));