[UX] Mobile navigation prevents access to chat history

## Issue Description

The left navigation is effectively unusable on mobile. The top navigation items consume most of the available vertical space, pushing the Chats section almost out of view so users cannot reliably reach their chat history.

The items out of view also fail to render - only a few of my recent chats are visible while the rest hang indefinitely, like the list never knows it's in view.

## Steps to Reproduce

1. Open Azure SRE Agent on a mobile device or resize the viewport to approximately 390 x 844 CSS pixels.
2. Open the left navigation.
3. Attempt to reach the Chats section and select an existing thread.

## Actual Behavior

The top navigation items fill the available space and the user's chat history is pushed below the visible area or cannot be reached practically.

## Expected Behavior

The navigation should default to collapsed on mobile. When opened, its contents should use a bounded, scrollable layout that keeps the Chats section and chat history reachable at supported viewport heights.

## Impact

Blocks a core mobile navigation workflow: returning to an existing conversation.

Reproduce the issue at approximately 390 × 844 CSS pixels by opening the left navigation and attempting to reach an existing chat. Inspect the mobile navigation implementation and verify that it defaults to collapsed, uses a bounded scrollable layout when opened, and keeps the Chats section and chat history reachable.

Written by the indexing model from the issue text.