Add flatten mode to topic tree

[bracyw]

Description

Add a flatten mode toggle to the graph sidebar topic tree. When active, the tree renders as a flat list with nodes displayed as (system name)...(base topic name) instead of the nested hierarchy. This is a building block for other views like a selected-topics-only mode.

Acceptance Criteria

• Flatten mode toggle is visible in the topic tree header
• Activating flatten mode renders a flat node list (no nesting) showing (system name)...(base topic name)
• Node selection behavior unchanged — clicking toggles selection on/off
• Toggling flatten mode off returns to normal tree with selection state preserved

Proposed Solution

Add a flattenMode signal to the sidebar components. When active, derive a flat node list from the existing tree built by dataTypesToNodes() in utils/dataTypes.utils.ts. Add the toggle button near the existing search input in the sidebar header.