feat: add settings preset save/load

[henleyb]

Summary

Adds a Presets section to the sidebar for saving, loading, exporting, and importing named settings presets. Presets are stored in localStorage under the key bumpmesh_presets_v1. Export/import as JSON allows sharing between devices.

What's captured

• All sliders: amplitude, scale U/V, offset U/V, rotation, texture smoothing, seam blend, seam band width, cap angle, boundary falloff, bottom/top angle limits, resolution, output triangles
• Mapping mode (Triplanar, Cubic, Cylindrical, etc.)
• Scale lock state
• Symmetric displacement toggle
• Active built-in texture name (restored by clicking the matching swatch)

What's intentionally out of scope

• Surface paint masks (complex geometry data, not worth serialising in v1)
• Custom uploaded textures (can't serialise binary image data cleanly to JSON)

Implementation notes

• New js/presets.js module — UI + localStorage, decoupled from app internals via getState/applyState callbacks
• main.js: hoists _applyScaleV to module scope (was an inline closure) so the preset restore path can use it directly; adds getSettingsSnapshot/applySettingsSnapshot and the initPresets call after wireEvents()
• index.html: adds an empty <section id="presets-section"> placeholder populated at runtime
• style.css: preset UI styles using the existing CSS variables (--accent, --surface2, --border, --radius)
• i18n: English + German translations added. The other 6 languages fall back to English via the existing fallback mechanism — happy to add more translations if preferred

Testing

Tested locally in Brave and Chrome on Windows. Verified:

• Save/load round-trip restores all captured settings correctly (including the log-scale transform on scale U/V)
• Scale lock state restores before scale values, preventing lock-toggle side effects
• Export downloads a readable JSON file; import merges with existing presets
• Rotation gizmo and all other upstream features continue to work unchanged