feat: add session activity indicator in sidebar

[WanderingHumanid]

Description

Adds a visual activity indicator to the sidebar to clearly identify the currently active chat session.

Closes #260

Changes

• Added a purple activity indicator dot for the active session in the sidebar
• Preserved existing active session highlighting and styling
• Used absolute positioning and adjusted padding to prevent layout shifts when switching sessions
• Marked the indicator as aria-hidden="true" since it is a decorative visual element

Verification

1. Open LocalMind and create or load multiple chat sessions.
2. Select a session from the sidebar.
3. Verify that the purple activity indicator appears next to the active session.
4. Switch between sessions and verify that the indicator moves accordingly.
5. Confirm that session titles and icons remain properly aligned without layout shifts.

Before

After

[vercel]

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