refactor: add EnvConfig for centralized env var management (#556)

## Summary

Phase 1+2 of env var refactoring. Introduces `EnvConfig` struct that
reads all known environment variables once at startup, enabling
testable, parallel-safe code.

## Problem

- 235 direct `std::env` calls across 7 crates, 140 are
`set_var`/`remove_var`
- `env::set_var` is `unsafe` since Rust 1.83
- Tests need `#[serial]` or `#[ignore]` due to global env mutation
- No single source of truth for supported env vars

## Changes

### New: `vite_shared::EnvConfig`
- Central struct documenting all 11 known env vars
- `EnvConfig::from_env()` — read from real environment (production)
- `EnvConfig::for_test()` — sensible defaults, zero env access
- `EnvConfig::for_test_with_home(path)` — convenience for temp dir tests
- Supports struct update syntax: `EnvConfig { is_ci: true,
..EnvConfig::for_test() }`

### Updated: `home.rs`
- New `get_vite_plus_home_with_config(&EnvConfig)` — config-based, no
env access
- Legacy `get_vite_plus_home()` preserved for backward compat
- Removed `#[ignore]` test, replaced with parallel-safe config test

### Updated: `tracing.rs`
- New `init_tracing_with_config(&EnvConfig)` — config-based
- Legacy `init_tracing()` preserved
- Extracted shared `init_tracing_inner()`

## Test Results
- `cargo test -p vite_shared`: 19 passed + 2 doc-tests ✅
- `cargo check` (full project): compiles ✅

## Next Phases
- Phase 3: Thread through `vite_install` + `vite_js_runtime`
- Phase 4: Thread through `vite_global_cli` (biggest change)
- Phase 5: Remove remaining `unsafe env::set_var`

Author: fengmk2

crates/vite_global_cli/src/commands/env/bin_config.rs

@@ -112,20 +112,16 @@ impl BinConfig {
 
 #[cfg(test)]
 mod tests {
-    use serial_test::serial;
     use tempfile::TempDir;
 
     use super::*;
 
     #[tokio::test]
-    #[serial]
     async fn test_save_and_load() {
         let temp_dir = TempDir::new().unwrap();
-
-        // SAFETY: This test runs in isolation with serial_test
-        unsafe {
-            std::env::set_var("VITE_PLUS_HOME", temp_dir.path());
-        }
+        let _guard = vite_shared::EnvConfig::test_guard(
+            vite_shared::EnvConfig::for_test_with_home(temp_dir.path()),
+        );
 
         let config = BinConfig::new(
             "tsc".to_string(),
@@ -142,22 +138,14 @@ mod tests {
         assert_eq!(loaded.package, "typescript");
         assert_eq!(loaded.version, "5.0.0");
         assert_eq!(loaded.node_version, "20.18.0");
-
-        // Clean up env var
-        unsafe {
-            std::env::remove_var("VITE_PLUS_HOME");
-        }
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_find_by_package() {
         let temp_dir = TempDir::new().unwrap();
-
-        // SAFETY: This test runs in isolation with serial_test
-        unsafe {
-            std::env::set_var("VITE_PLUS_HOME", temp_dir.path());
-        }
+        let _guard = vite_shared::EnvConfig::test_guard(
+            vite_shared::EnvConfig::for_test_with_home(temp_dir.path()),
+        );
 
         // Create configs for typescript (tsc, tsserver)
         let tsc = BinConfig::new(
@@ -197,22 +185,14 @@ mod tests {
 
         let nonexistent_bins = BinConfig::find_by_package("nonexistent").await.unwrap();
         assert!(nonexistent_bins.is_empty());
-
-        // Clean up env var
-        unsafe {
-            std::env::remove_var("VITE_PLUS_HOME");
-        }
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_delete() {
         let temp_dir = TempDir::new().unwrap();
-
-        // SAFETY: This test runs in isolation with serial_test
-        unsafe {
-            std::env::set_var("VITE_PLUS_HOME", temp_dir.path());
-        }
+        let _guard = vite_shared::EnvConfig::test_guard(
+            vite_shared::EnvConfig::for_test_with_home(temp_dir.path()),
+        );
 
         let config = BinConfig::new(
             "tsc".to_string(),
@@ -235,29 +215,16 @@ mod tests {
 
         // Delete again should not error
         BinConfig::delete("tsc").await.unwrap();
-
-        // Clean up env var
-        unsafe {
-            std::env::remove_var("VITE_PLUS_HOME");
-        }
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_load_nonexistent() {
         let temp_dir = TempDir::new().unwrap();
-
-        // SAFETY: This test runs in isolation with serial_test
-        unsafe {
-            std::env::set_var("VITE_PLUS_HOME", temp_dir.path());
-        }
+        let _guard = vite_shared::EnvConfig::test_guard(
+            vite_shared::EnvConfig::for_test_with_home(temp_dir.path()),
+        );
 
         let loaded = BinConfig::load("nonexistent").await.unwrap();
         assert!(loaded.is_none());
-
-        // Clean up env var
-        unsafe {
-            std::env::remove_var("VITE_PLUS_HOME");
-        }
     }
 }