fix(config): improve Gemini merge and remove duplicate logging

Address multiple audit findings:

1. config_merge.rs:701 - Gemini merge now initializes env from common
   config when custom config has no env field, ensuring common API keys
   and base URLs are applied correctly.

2. Remove duplicate logging - merge functions now only return warnings,
   callers (live.rs, proxy.rs) handle logging in one place.

3. live.rs:169 - Unify error message style to plain English descriptions
   instead of error codes embedded in messages.

Changes:
- Add fallback to initialize env from common_env when missing
- Remove log::warn! calls from merge functions
- Use descriptive error messages consistently

src-tauri/src/config_merge.rs

@@ -617,12 +617,10 @@ fn merge_claude_config(custom_config: &JsonValue, common_snippet: &str) -> Merge
     let common_value: JsonValue = match serde_json::from_str(common_snippet) {
         Ok(v) => v,
         Err(e) => {
-            log::warn!(
+            // Return warning without logging - caller will log if needed
-                "Claude common config snippet parse error, skipping merge: {e}"
-            );
             return MergeResult {
                 config: custom_config.clone(),
-                warning: Some(format!("COMMON_CONFIG_PARSE_ERROR: {e}")),
+                warning: Some(format!("Claude common config parse error: {e}")),
             };
         }
     };
@@ -643,8 +641,8 @@ fn merge_codex_config(custom_config: &JsonValue, common_snippet: &str) -> MergeR
             let (merged_toml, error) =
                 compute_final_toml_config_str(config_str, common_snippet, true);
             if let Some(e) = error {
-                log::warn!("Codex common config merge warning: {e}");
+                // Return warning without logging - caller will log if needed
-                warning = Some(format!("CODEX_TOML_MERGE_WARNING: {e}"));
+                warning = Some(format!("Codex TOML merge warning: {e}"));
             }
             obj.insert("config".to_string(), JsonValue::String(merged_toml));
         }
@@ -667,12 +665,10 @@ fn merge_gemini_config(custom_config: &JsonValue, common_snippet: &str) -> Merge
     let common_value: JsonValue = match serde_json::from_str(common_snippet) {
         Ok(v) => v,
         Err(e) => {
-            log::warn!(
+            // Return warning without logging - caller will log if needed
-                "Gemini common config snippet parse error, skipping merge: {e}"
-            );
             return MergeResult {
                 config: custom_config.clone(),
-                warning: Some(format!("COMMON_CONFIG_PARSE_ERROR: {e}")),
+                warning: Some(format!("Gemini common config parse error: {e}")),
             };
         }
     };
@@ -699,6 +695,7 @@ fn merge_gemini_config(custom_config: &JsonValue, common_snippet: &str) -> Merge
     };
 
     // Merge only the env field
+    // If custom config has env, merge common into it; otherwise initialize with common env
     if let Some(merged_obj) = merged_config.as_object_mut() {
         if let Some(merged_env) = merged_obj.get_mut("env") {
             if let Some(merged_env_obj) = merged_env.as_object_mut() {
@@ -709,6 +706,9 @@ fn merge_gemini_config(custom_config: &JsonValue, common_snippet: &str) -> Merge
                 }
                 *merged_env = JsonValue::Object(final_env);
             }
+        } else if !common_env.is_empty() {
+            // Custom config has no env field - initialize with common env
+            merged_obj.insert("env".to_string(), JsonValue::Object(common_env));
         }
     }
 

src-tauri/src/services/provider/live.rs

@@ -167,17 +167,16 @@ fn write_live_snapshot_internal(
         }
         AppType::Codex => {
             let obj = config_to_write.as_object().ok_or_else(|| {
-                AppError::Config(
+                AppError::Config("Codex provider settings_config must be a JSON object".to_string())
-                    "CODEX_CONFIG_NOT_OBJECT: settings_config must be a JSON object".to_string(),
-                )
             })?;
             let auth = obj.get("auth").ok_or_else(|| {
-                AppError::Config(
+                AppError::Config("Codex provider settings_config missing 'auth' field".to_string())
-                    "CODEX_CONFIG_MISSING_AUTH: settings_config missing 'auth' field".to_string(),
-                )
             })?;
             let config_str = obj.get("config").and_then(|v| v.as_str()).ok_or_else(|| {
-                AppError::Config("CODEX_CONFIG_MISSING_CONFIG: settings_config missing 'config' field or not a string".to_string())
+                AppError::Config(
+                    "Codex provider settings_config missing 'config' field or not a string"
+                        .to_string(),
+                )
             })?;
 
             let auth_path = get_codex_auth_path();