feat(ui): add dedicated Agent subagent rendering with nested tool call display

Render Agent/Explore/Plan tool calls in a visually distinct collapsible
container with colored left border, replacing the generic tool card. Parse
subagent JSONL transcripts from {sessionId}/subagents/ to extract and
display the actual tool calls (Bash, Read, Grep, etc.) the subagent
executed, reusing the existing ToolCallPart for consistent appearance.

- Add AgentToolCallPart component with collapsible body, prompt section,
  execution stats, and nested tool call list via render prop injection
- Add AgentExecutionStats and AgentToolCall types (Rust + TypeScript)
- Parse toolUseResult.agentId to locate and read subagent JSONL files
- Validate agentId against path traversal before filesystem access
- Pass agentStats through adapter for both ID-matched and positional
  tool result pairing
- Strip agentStats in nested render to prevent recursive Agent expansion
- Add i18n keys for agent UI labels across all 10 languages

src-tauri/src/parsers/cline.rs

@@ -370,6 +370,7 @@ fn parse_user_message_parts(content: &serde_json::Value) -> UserMessageParts {
                 tool_use_id: None,
                 output_preview: Some(truncate_str(&output, 2000)),
                 is_error,
+                agent_stats: None,
             });
 
             // If the tool result also contains <feedback>, extract it
@@ -565,6 +566,7 @@ fn parse_content_blocks(content: &serde_json::Value) -> Vec<ContentBlock> {
                             tool_use_id,
                             output_preview,
                             is_error,
+                            agent_stats: None,
                         });
                     }
                     "thinking" => {