DRY: merge duplicated topic-edge lookup helpers

[lowhung]

Summary

Unify duplicate topic-edge lookup helpers in details panel into one directional lookup utility.

Why

find_source_node_for_topic and find_target_node_for_topic are mirrored traversals over the edge list. A single helper reduces branching duplication and future maintenance overhead.

Affected files

• src/ui/panels.rs

Extraction plan

• Introduce enum (Incoming/Outgoing) and helper like:
  find_connected_node_for_topic(graph, topic, direction, node_name) -> Option<String>
• Replace both existing helper usages in Inputs/Outputs sections.
• Remove redundant old helpers.
• Add tests covering both directions and no-match behavior.

Risk

Low to medium. Touches edge-selection UX in details panel; incorrect direction mapping would break clickable topic navigation.

Acceptance criteria

• Single directional helper replaces both lookup functions.
• Clicking input/output topics still selects expected edge.
• Tests pass.