v026 — Theme foundation

Inter Variable font embedded, Phosphor icon set wired, full Miro palette ported as Rust constants in bse-ui/src/theme/colors.rs. apply_bse_theme(ctx, ThemeMode) installs the whole stack in one call. Motion tokens (durations, easings, spring presets) ready for the next milestones. This is plumbing — the app already looks subtly different but the visible payoffs land starting v027.