Expose on-demand activity details for long-running turns

[saphid]

Problem

The chat timeline can remain on Working for … for a long time without showing whether the request was dispatched, whether any provider events have arrived, or what the last feedback was. That makes a slow provider, an active tool call, and a genuinely stuck turn look the same.

This is adjacent to reports such as #2644 and #2778, but the proposal is diagnostic UI rather than a claim to fix those provider-specific hangs.

Proposed behavior

Add a collapsed-by-default Details disclosure to the live working row on web and native mobile. When opened, it shows:

• request age and timestamp;
• last provider/assistant feedback age and timestamp;
• active turn ID;
• provider-event and tool-call counts;
• the active turn activity stream, newest first;
• an explicit no-feedback-yet state.

The projection should stay scoped to the active turn and avoid serializing arbitrary provider payloads. Only short, selected text fields should be surfaced.

Why on demand

This keeps the normal chat timeline quiet while preserving enough evidence to distinguish “request sent, provider silent” from “provider is still producing events/tool activity.” The same diagnostic should be available on web and mobile so the phone is not a second-class debugging surface.