Make text layers easier to drag

[dariomory]

Fixes #2

hitTestLayer rejected any pixel with alpha below 10. A text raster is mostly transparent between the glyphs, so unless you grabbed a letter stroke exactly, the layer slipped out from under the cursor. Text layers are now picked by their bounding box instead.

+    // Text rasters are mostly transparent between glyphs, so a per-pixel alpha
+    // test makes them slip out from under the cursor. Their bounding box is the
+    // grabbable area instead.
+    if (layer.kind === "text" || layer.role === "text") return layer;

Image and paint layers keep the per-pixel test, so clicking through the transparent part of a photo layer still behaves as before.

Trade-off worth knowing: a text layer now captures clicks across its whole box, gaps included. Picking a layer underneath means clicking outside the text bounds. That is the usual bargain for making text grabbable, and it is the behaviour the issue asks for.

Verification

Driven in a browser against the running editor: place a text layer, switch to Move, drag starting from a gap between two letters, then sample the canvas. The origin reads white and the drop point reads dark, so the layer followed the drag. Before the change that same drag did nothing.

pnpm test (21) and pnpm exec tsc --noEmit are clean.

Branched off master and independent of #13, so the two can merge in either order.