fix: silent fallback to default on any server_config.yml error

[YuevUwU]

Changes

• Use default if server_config.yml is not found
  with warn log and yaml form of default (as example for users)

  2026-03-27T16:01:57.614117Z  WARN phira_mp_server::server: server_config.yml is not found.
  Fallback to default config:
  ```yaml
  monitors:
  - 2
  ```
  2026-03-28T03:37:50.922670Z  INFO phira_mp_server::server: User IDs allowed to monitor matches: [2]

• Terminate if raised other io::Error or serde_yaml::Error

  Error: Failed to parse server_config.yml
  
  Caused by:
      Is a directory (os error 21)
  

  Error: Failed to parse server_config.yml
  
  Caused by:
      missing field `monitors`
  

  Error: Failed to parse server_config.yml
  
  Caused by:
      monitors[0]: invalid type: string "41321", expected i32 at line 1 column 12
  

• Log current monitors config

   2026-03-28T03:37:41.402080Z  INFO phira_mp_server::server: User IDs allowed to monitor matches: [41321]
  

• TcpListener -> Server is now TryFrom
• Extract "server_config.yml" to a const string

chore: log config.monitors with info!

Past Problem

let config: ServerConfig = File::open("server_config.yml")  // Result<ServerConfig, io::Error>
    .ok()
    .and_then(|f| serde_yaml::from_reader(f).ok())  // 
    .unwrap_or_default();  // Suppress all error.
// User don't know if it's okay or failed

It's now no longer suppress errors, and users can be aware of their current configuration. (just like phira-monitor)
Mivik can still use without server_config.yml file in phira-mp-server::server::CONFIG_PATH.
But other error is now considered as hard error.

Extra Consideration (NOT in this PR)

In my environment, the default max log level is error.
This patch could be considered to "RUST_LOG=warn by default", and still check RUST_LOG first.
This is better than simply writing RUST_LOG=info in the README.md.
(Edge Case: If someone set RUST_LOG as invalid string, it still apply and suppress, and won't use warn by default.)

diff --git a/phira-mp-server/src/main.rs b/phira-mp-server/src/main.rs
index d50916f..80bec15 100644
--- a/phira-mp-server/src/main.rs
+++ b/phira-mp-server/src/main.rs
@@ -65,9 +65,11 @@ pub fn init_log(file: &str) -> Result<WorkerGuard> {
                 .with_filter(LevelFilter::DEBUG),
         )
         .with(
-            fmt::layer()
-                .with_writer(std::io::stdout)
-                .with_filter(EnvFilter::from_default_env()),
+            fmt::layer().with_writer(std::io::stdout).with_filter(
+                EnvFilter::builder()
+                    .with_default_directive(LevelFilter::WARN.into())
+                    .from_env_lossy(),
+            ),
         )
         .with(
             filter::Targets::new()