feat(web-app): sidebar status dots for thread activity

[yanalialiuk]

Summary

• Add a pulsing dot next to sidebar chat titles while a thread is generating (streaming or pending tool calls)
• Add a static dot when a background reply completes before the user opens that conversation
• Clear unread state when the thread is opened; clean up on thread deletion

Test plan

• yarn tsc --noEmit in web-app passes
• Start a reply in chat A, switch to chat B — chat A shows a pulsing dot on the left
• Wait for reply to finish while still on chat B — dot becomes static (unread)
• Open chat A — dot disappears
• Delete a thread with an unread dot — no stale state remains

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