configurable stream timeout, show per-call timer in status bar

Stream chunk timeout is now api_stream_timeout_secs in config
(default 60s). Status bar shows total turn time and per-call
time with timeout: "thinking... 45s, 12/60s".

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -305,7 +305,7 @@ impl SseReader {
     pub(crate) fn new(ui_tx: &UiSender) -> Self {
         Self {
             line_buf: String::new(),
-            chunk_timeout: Duration::from_secs(60),
+            chunk_timeout: Duration::from_secs(crate::config::get().api_stream_timeout_secs),
             stream_start: Instant::now(),
             chunks_received: 0,
             sse_lines_parsed: 0,