feat(web): inline tool activity cards with auto-collapsing (#376)

* feat(web): inline tool activity cards with auto-collapsing

Add Claude/Codex-style inline tool activity cards to the web UI that
show tool execution progress directly in the chat conversation.

While processing:
- Animated thinking dots with message text (e.g. "Calling LLM...")
- Individual tool cards with live spinner and elapsed timer
- Cards show tool name, duration, and expandable output preview

After response arrives:
- Activity group auto-collapses to "Used N tools (Xs)"
- Click summary to expand and see individual tool cards
- Click card header to see tool output in monospace

Also includes:
- "Calling LLM..." thinking status from dispatcher (all channels)
- 5-minute max timer guard to prevent leaks on dropped SSE
- Handles parallel tools, same tool twice, failures, thread switching

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix(web): use frozen duration for completed tools in activity summary

The collapsed activity summary was showing inflated total duration
because finalizeActivityGroup() recalculated elapsed time from
Date.now() for already-completed tools. Now each tool card stores
its final duration at completion time and the summary uses that
frozen value instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/agent/dispatcher.rs

@@ -212,6 +212,15 @@ impl Agent {
                 );
             }
 
+            let _ = self
+                .channels
+                .send_status(
+                    &message.channel,
+                    StatusUpdate::Thinking("Calling LLM...".into()),
+                    &message.metadata,
+                )
+                .await;
+
             let output = match reasoning.respond_with_tools(&context).await {
                 Ok(output) => output,
                 Err(crate::error::LlmError::ContextLengthExceeded { used, limit }) => {