Per-glyph layers with Figma-style move tool

[aurpelai]

Summary

Adds a per-glyph layer system (max 5 layers) so users can reposition compound parts like umlauts or strikes without redrawing the base letter. Each layer has its own pixel buffer, offset, ink tint, visibility, lock, and preview-vs-white toggle. The move tool now hit-tests the topmost inked visible layer under the cursor (Figma-style), with Alt+click to cycle through stacked layers and a layer-panel multi-select for whole-glyph drags. The export pipeline is unaffected externally — layers are flattened to one bitmap before atlas packing.

This is Stage A + Stage B in a single PR per the layer-system plan. A small Stage B follow-up is queued to drop the legacy top-level pixel fields from the in-memory `Glyph` type (see "Follow-ups" below).

Highlights

• Data model. New `Layer` interface with id/name/pixels/width/height/xoffset/yoffset/visible/preview/color/locked. `Glyph.layers: Layer[]` is the new authoritative source; legacy top-level pixel fields are kept on the type and on disk during this PR so first-time rollback to v1 stays safe.
• Dexie v1→v2 migration. Wraps each existing glyph's bitmap into `layers[0]` via `makeBaseLayerFromBitmap`. Legacy fields preserved.
• Layer helpers in `src/core/project/layers.ts` — pure functions for flatten, hit-test, cycle-hit, union bounds, add/remove/reorder/update layer mutators, trim-to-ink, sync-legacy-fields. All covered by unit tests.
• Layer-aware editor. PixelEditor renders each visible layer bottom-up (in its tint or white based on the preview toggle), pencil/eraser target the active layer, move tool hit-tests via `hitTestLayer` and shows the grab outline around just the picked layer. Alt+click cycles, multi-select moves several layers together as one undo step.
• LayerPanel UI. Right-docked side rail with eye/preview/lock/delete per row, `+` button capped at 5 layers, Shift/Cmd-click for multi-select. Heading and color match the GlyphList style.
• Trim-to-ink runs on every paint and on project load so layer bounds always hug their visible ink (repairs the misaligned grab outline reported during testing).
• Palette. Index 0 is white (base layer renders identically to the final exported ink); indices 1–4 are amber/cyan/lime/violet so additional layers stand out.

Test plan

• Open an existing v1 project — migration runs once, every glyph has `layers[0]` and renders identically to before.
• Import a TTF — every glyph has one "Base" layer; editor behaviour matches the pre-layer baseline.
• On a glyph like `ä`: add a second layer, draw two dots, toggle preview off → confirm the dots render in white (final look).
• Hide the umlaut layer → only the base `a` renders. Re-show it.
• Move tool: hover the dots → grab outline wraps just the umlaut. Drag — only the dots move; the base stays put.
• Alt+click on stacked layers to cycle the grab target.
• Shift-click two layer rows in the panel → drag in the canvas moves both layers together as one undo step.
• Add 4 more layers, confirm `+` button disables at 5.
• Lock a layer, confirm pencil and move both no-op on it.
• Export atlas + .fnt, confirm the flattened result is byte-identical to a single-layer baseline.
• Undo/redo across paint + move + layer add/remove.
• Reload the page — every layer's pixels, visibility, preview, color, name, lock state survives the IndexedDB round-trip.

Follow-ups (not in this PR)

1. Drop the legacy `Glyph` fields. `pixels`/`width`/`height`/`xoffset`/`yoffset` are still optional on the type. Audited as ~8 files to convert (PreviewFloat, GlyphThumbnail, GlyphList forwarders, PixelEditor layout, ImportWizard, EditorScreen, glyphs.ts, portable.ts) plus a v2→v3 strip migration. Worth doing as a separate PR after this bakes.
2. Brush highlight tinting. The plan called for tinting the cursor outline with the active layer's color when `preview=true`; not yet implemented (still always cyan/orange).

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