feat: support stroke-dashoffset (dash phase)

[softmarshmallow]

Summary

Add support for stroke-dashoffset (dash phase) across the engine, SVG import, serialization, and editor UI.

Currently StrokeDashArray is a plain Vec<f32> of alternating dash/gap lengths with no offset. SVG's stroke-dashoffset attribute is silently dropped during import.

Motivation

• stroke-dashoffset is a basic SVG stroke property — dropping it causes visual fidelity loss on import.
• Dash offset is essential for animated dashed strokes (e.g. "marching ants" selection, loading spinners).
• Design tools (Figma, Illustrator) expose dash offset as an authorable property.

Scope

Core engine (crates/grida-canvas)

• Add a dash_offset: f32 field to the stroke style schema (alongside stroke_dash_array)
• Wire the offset through to the Skia PathEffect::dash call (the second argument is phase)
• Update StrokeDashArray or the stroke style to carry the offset
• Serialization: .grida (JSON + FlatBuffers) roundtrip for the new field

SVG import (crates/grida-canvas/src/svg)

• Parse stroke-dashoffset from usvg (path.stroke().dashoffset()) during from_usvg conversion
• Store it in SVGStrokeAttributes and propagate to the node schema

SVG export

• Emit stroke-dashoffset when non-zero

Editor UI (editor/)

• Expose dash offset in the stroke properties panel (next to dash array)
• Allow interactive editing (drag / numeric input)

References

• SVG spec: https://www.w3.org/TR/SVG11/painting.html#StrokeDashoffsetProperty
• Skia: SkDashPathEffect::Make(intervals, count, phase)
• usvg: Stroke::dashoffset() returns f32