feat: add glow edge system with directional selection animation

Custom bezier edges with gradient glow for the flow editor:
- Default: subtle white/gray gradient with soft glow
- Downstream (cyan): animated flowing dashes from selected node through subtree
- Upstream (amber): animated flow from selected node back to root
- Cross-reference: dashed cyan with arrow markers
- SVG gradient + filter defs for performant rendering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-03-09-glow-edge-design.md

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+# Glow Edge System — Flow Editor
+
+> **Date:** 2026-03-09
+
+## Overview
+
+Replace flat `smoothstep` edges in the troubleshooting flow editor with custom bezier edges featuring gradient strokes, soft glow, and directional animation on node selection.
+
+## Default Edges (no selection)
+
+- **Curve type:** Bezier (replacing right-angled `smoothstep`)
+- **Stroke:** ~1.5px, subtle white/gray gradient with soft `drop-shadow` glow
+- **Feel:** Clean, understated, dark-mode friendly
+
+## Selected Node — Downstream Edges
+
+- **Color:** Cyan brand gradient (`#06b6d4` → `#22d3ee`)
+- **Animation:** Flowing dash animation moving downward (`stroke-dashoffset` keyframe)
+- **Scope:** All edges from selected node through entire subtree (children, grandchildren, etc.)
+- **Glow:** Soft cyan `drop-shadow` filter
+- **Stroke:** 2px
+
+## Selected Node — Upstream Edges
+
+- **Color:** Amber gradient (`#f59e0b` → `#fbbf24`)
+- **Animation:** Softer pulse/breathing opacity animation moving upward toward root
+- **Scope:** All edges from selected node back to root
+- **Glow:** Soft amber `drop-shadow` filter
+- **Stroke:** 2px
+
+## Cross-reference Edges
+
+- Keep dashed + animated + cyan with arrows
+- Use new bezier curves + glow treatment
+
+## Implementation
+
+- One custom `GlowEdge` component registered as the default edge type in React Flow
+- `useTreeLayout` passes `edgeState: 'default' | 'downstream' | 'upstream'` in edge data based on `selectedNodeId`
+- SVG `linearGradient` + `filter` defined once in a `<defs>` block
+- CSS keyframe animation for flowing dash effect
+
+## Files Touched
+
+- **New:** `frontend/src/components/tree-editor/GlowEdge.tsx` (~60 lines)
+- **Modified:** `useTreeLayout.ts` — add ancestor/descendant calculation, edge state
+- **Modified:** `FlowCanvas.tsx` — register custom edge type
+- **Modified:** `index.css` — keyframe animation for flowing dashes