feat: add per-row extension points inside sidebar chat list x-for loop

[Krashnicov]

Summary

Adds two new x-extension points inside the x-for loop in chats-list.html:

• sidebar-chat-item-start — injected immediately after the opening chat-container div, before the chat label
• sidebar-chat-item-end — injected after the close button, before the closing chat-container div

Motivation

Previously only sidebar-chats-list-start and sidebar-chats-list-end existed, both placed outside the x-for loop. Plugins that need to inject UI into individual chat rows had no framework-sanctioned way to do so.

This forced third-party plugins to work around the gap by:

• Monkey-patching internal store methods (chatsStore.applyContexts, chatsStore.selectChat)
• Using MutationObserver + index-based DOM scanning (contexts[index]) to locate rows
• Calling imperative createElement / insertBefore instead of declarative Alpine bindings

All three patterns are fragile — they break silently when internal store method names or DOM structure changes.

Solution

With sidebar-chat-item-start / sidebar-chat-item-end inside the x-for loop, extension HTML has access to the reactive Alpine context object (including context.id, context.name, context.running, context.project, etc.) through the normal Alpine scope chain.

This enables purely declarative per-row plugin UI:

<!-- No DOM scanning, no monkey-patching needed -->
<span x-show="context.running" class="my-running-dot"></span>

Change

Minimal — 2 lines added to webui/components/sidebar/chats/chats-list.html. No logic changes, no style changes, no new dependencies.

Backwards Compatibility

Fully backwards compatible. Existing sidebar-chats-list-start and sidebar-chats-list-end extension points are unchanged. The new points are no-ops when no plugin registers extensions for them.