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/src/usage_script.rs

@@ -269,7 +269,11 @@ async fn send_http_request(config: &RequestConfig, timeout_secs: u64) -> Result<
 
     if !status.is_success() {
         let preview = if text.len() > 200 {
-            format!("{}...", &text[..200])
+            let mut safe_cut = 200usize;
+            while !text.is_char_boundary(safe_cut) {
+                safe_cut = safe_cut.saturating_sub(1);
+            }
+            format!("{}...", &text[..safe_cut])
         } else {
             text.clone()
         };