v0.8.0

Valence v0.8.0 — Bun 1.4, PubChem record, gallery, teaching presets

Features: PubChem MMFF94 data + CID name, full PubChem record
collapsible, atom-label distance fade, pi-system highlighting, Phase 1-3
teaching views (labels, presets, space-filling, shareable links).

Fixes: per-bond pi tubes (fused rings), kekulize guard for PubChem,
pi-system geometry corrections, VdW radii, hydrogen s-orbital.

UI: Build-tab fold, Local MMFF94 badge, screenshot gallery, Bun 1.4
migration, adaptive label colors, white default background.

See https://github.com/exergonic/valence/compare/v0.7.0...v0.8.0