Fix/Resolve panic issues in proxy-related code (#560)

* fix(proxy): change default port from 5000 to 15721

Port 5000 conflicts with AirPlay Receiver on macOS 12+.
Also adds error handling for proxy toggle and i18n placeholder updates.

* fix(proxy): replace unwrap/expect with graceful error handling

- Handle HTTP client initialization failure with no_proxy fallback
- Fix potential panic on Unicode slicing in API key preview
- Add proper error handling for response body builder
- Handle edge case where SystemTime is before UNIX_EPOCH

* fix(proxy): handle UTF-8 char boundary when truncating request body log

Rust strings are UTF-8 encoded, slicing at a fixed byte index may cut
in the middle of a multi-byte character (e.g., Chinese, emoji), causing
a panic. Use is_char_boundary() to find the nearest safe cut point.

* fix(proxy): improve robustness and prevent panics

- Add reqwest socks feature to support SOCKS proxy environments
- Fix UTF-8 safety in masked_key/masked_access_token (use chars() instead of byte slicing)
- Fix UTF-8 boundary check in usage_script HTTP response truncation
- Add defensive checks for JSON operations in proxy service
- Remove verbose debug logs that could trigger panic-prone code paths

src-tauri/Cargo.toml

@@ -37,7 +37,7 @@ tauri-plugin-deep-link = "2"
 dirs = "5.0"
 toml = "0.8"
 toml_edit = "0.22"
-reqwest = { version = "0.12", features = ["rustls-tls", "json", "stream"] }
+reqwest = { version = "0.12", features = ["rustls-tls", "json", "stream", "socks"] }
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "time", "sync"] }
 futures = "0.3"
 async-stream = "0.3"