feat: add support for deleting chats

[adranjan4]

Summary

• Add confirmation dialogs for deleting a single chat and clearing all chats
• Add multi-select mode so users can choose and delete specific chats
• Persist deleted thread IDs so chats do not reappear after reload if backend sync races
• Reuses existing Redux + deleteThread API already on deep-agent

Closes #12
Supersedes #66 (that PR targeted main)

Test plan

• Hover a chat and click delete — confirmation appears
• Cancel delete — chat remains
• Confirm delete — chat is removed from sidebar
• Delete the active chat — navigates to another chat or home
• Click Select — checkboxes appear; select multiple chats and Delete
• Cancel selection mode — returns to normal sidebar
• Click Clear all — confirmation shows chat count; confirm removes all and navigates home
• Refresh page — deleted chats stay deleted

[NP-compete]

The current template agent does not support this feature.

[adranjan4]

Thanks for linking #12 - could you also assign me to it so the Check Issue Link CI can pass?

On the agent point: understood. This PR is UI-only and reuses deleteThread (DELETE /threads/{id}) plus a localStorage fallback so deleted chats don’t reappear in the sidebar. If template-agent doesn’t implement thread delete yet, server-side deletion won’t fully work.

Happy to follow your preference:

1. Keep this as UI soft-delete for now (sidebar + local persistence), or
2. I can open a companion PR on template-agent to add delete-thread support and wire this UI to it.

Which would you prefer?

[NP-compete]

Thanks for linking #12 - could you also assign me to it so the Check Issue Link CI can pass?

On the agent point: understood. This PR is UI-only and reuses deleteThread (DELETE /threads/{id}) plus a localStorage fallback so deleted chats don’t reappear in the sidebar. If template-agent doesn’t implement thread delete yet, server-side deletion won’t fully work.

Happy to follow your preference:

1. Keep this as UI soft-delete for now (sidebar + local persistence), or
2. I can open a companion PR on template-agent to add delete-thread support and wire this UI to it.

Which would you prefer?

Lets create the companion PR, and in the meanwhile please change the PR to draft state.