fix: route warnings to file and add in-app toast notifications

- Replace all eprintln! with tracing::warn! to avoid TUI corruption
  via stderr writes during alternate screen mode
- Route tracing output to ~/.local/share/zesdex/zesdex.log instead of
  stderr by configuring tracing_subscriber with a Mutex<File> writer
- Add render_toasts() widget: floating notification stack at top-right
  of the terminal, color-coded by severity (Info/Success/Warning/Error/
  Lesson), auto-expires after 5s, max 4 visible
- Apply to 12 files: stream parser, MCP client, subagent engine,
  state/rest, controller/input, app_config, provider, OAuth, agent_def,
  dto/chat/tool

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/dto/chat/tool.rs

@@ -21,7 +21,7 @@ pub fn sanitize_tool_arguments(args: &Value) -> Value {
             match serde_json::from_str::<Value>(s) {
                 Ok(v) => v,
                 Err(e) => {
-                    eprintln!(
+                    tracing::warn!(
                         "warning: tool argument is a JSON string but failed to parse: {}. Using raw string.",
                         e
                     );