WebUI (chat): show thinking/working indicator after send

[edwin-zvs]

This improves Web UI chat mode feedback right after submit:

• Show a lightweight inline “assistant · thinking” row (with a small spinner) while AgentStatus.active is true for the selected session. Auto-removes when the turn ends.
• Also surface the status banner (Working.. 3s) in chat mode by reusing renderEditorState with the session’s agent status, so users always see that something is happening.
• No changes to terminal mode behavior.

Implementation

• Leverage existing agent_status live events: on active=true ensureThinkingBanner(); on active=false removeThinkingBanner().
• Minimal CSS (spinner animation) and DOM adjustments; transcript logic remains unchanged.

Why

• Users lacked immediate post-submit feedback in chat mode when the agent was tool-running or composing. This adds clear, low-noise affordance without altering transcript semantics.

Manual check

• Chat send on smith session: optimistic user row → thinking banner appears immediately once AgentStatus.active arrives; replaced by assistant/tool rows when they stream; banner clears at turn end.
• Terminal mode unaffected; existing status row under the input remains as-is.

Follow-ups (optional)

• Replace ⏳ with an SVG spinner for higher-DPI crispness.
• Add a per-session top-right dot in the session list row synced with AgentStatus.active.