Graph view hardening: labels, fit-view, zoom controls, legend (+ write gate for #1180)

[ajianaz]

Requires uteke: v0.16.0+ (read) · add-connection gated on uteke#1180 fix · Epic: #289

What

• d3-force render upgrades: node labels on by default (halo text, 11-12px), size by degree, auto fit-view + center on load/filter change, zoom/pan controls cluster (＋ − ⤢), legend (memory vs isolated node, edge types), hover tooltip (author + time).
• Add-connection disabled with UpgradePrompt until server fix (uteke#1180) is released; remove-edge also gated (same FK domain) — read-only graph fully functional.

Why

Vision audit: 4 HIGHs (unlabeled 3-4px nodes, no legend, low-contrast edges, no fit/controls) — currently "looks broken".

Testing

• E2E: fit-view called on load/filter; legend toggles; tooltip content.
• Perf: synthetic 200-node layout < 2s to interactive; visual QA both themes.