Add WARZONE_HOME env var for separate user data directories

All data paths now use keystore::data_dir() which checks
WARZONE_HOME first, falls back to ~/.warzone.

This avoids the HOME override hack that breaks rustup/cargo.

Usage: WARZONE_HOME=/tmp/bob warzone init

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-client/src/storage.rs

@@ -13,8 +13,7 @@ pub struct LocalDb {
 
 impl LocalDb {
     pub fn open() -> Result<Self> {
-        let home = std::env::var("HOME").unwrap_or_else(|_| ".".into());
-        let path = std::path::Path::new(&home).join(".warzone").join("db");
+        let path = crate::keystore::data_dir().join("db");
         let db = sled::open(&path).context("failed to open local database")?;
         let sessions = db.open_tree("sessions")?;
         let pre_keys = db.open_tree("pre_keys")?;