feat(image-editor): wire Paint Bucket (G) — flood fill

[lyfuci]

Summary

Promotes the Paint Bucket (G) tool from stub to functional. Click on the
canvas → connected region of similar pixels gets filled with the
foreground color and lands as a new image-shape layer (non-destructive,
undoable).

Implementation

• New `src/lib/image-editor/flood-fill.ts` — 4-connected scanline flood
  fill on raw `ImageData` with a configurable channel-wise tolerance
  (0–128). `maskToDataUrl` paints the mask with the chosen color and
  serialises to a PNG data URL.
• `Canvas.tsx` exposes a new `onBucketClick(point)` callback. The fill
  itself runs in `ImageEditor.tsx` because it needs to re-render the
  full editor state at source resolution (not just the cropped preview)
  to operate on the true image.
• The fill bitmap is stored at SOURCE resolution so exports stay sharp;
  the layer's preview-pixel rect spans the full canvas.
• `OptionsBar` gains a Bucket variant: FG color picker + tolerance
  slider (default 32).
• Click on a transparent edge / out-of-bounds → empty mask → toast
  message rather than a phantom layer.
• Coord math handles an active crop: the click is in cropped-canvas
  preview pixels, so we shift back by the crop origin before scaling
  to source.

Removes `bucket` from `STUB_TOOLS`.

Branched from `main` (independent of #38).

Test plan

• Open an image, press G — Bucket variant appears in the options bar (color + tolerance).
• Click a uniform region — fills with FG color, new layer appears in the layer list.
• Tolerance slider changes the size of the filled region.
• Cmd+Z undoes the fill.
• Click an isolated speck → toast "Nothing to fill at that point".
• Switch FG color, fill again — second layer uses new color.
• Crop the image, then bucket-fill — fill respects the cropped view (no spillover into hidden pixels).
• Export PNG — fill is at source resolution (not blurry).
• Save .json project, reload — bucket layer round-trips.

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