Snapshot Diff: Skip highlighting for config nodes and clear previous glow

[andypalmi]

Description

When navigating to a global-config or flow-config node in the snapshot diff viewer, the previous node's glow highlight remains visible because config nodes have no visual presence on the SVG canvas and nothing triggers a new highlight to replace the old one.

Changes

• Add nodeCategory() helper to classify nodes as tab, group, subflow, global-config, flow-config, or regular node
• Add CONFIG_CATEGORIES constant for categories without SVG presence
• When navigating to a config node, call clearRendererHighlight() to remove the previous glow
• Show "Global Config" / "Flow Config" purple badge in the nav bar via currentGroupCategoryLabel computed property

Parent issue

Part of #7062