feat(image-editor): wire Gradient (G) — drag to define linear gradient

[lyfuci]

Summary

Promotes the Gradient tool from stub to functional. Drag from start to
end on the canvas → linear FG→BG gradient lands as a new image-shape
layer covering the full canvas.

Implementation

• New `Interaction` kind in `Canvas.tsx`: `gradient-drawing` (start +
  current end in canvas-pixel space). mousedown → start; mousemove →
  update end; mouseup → `onCommitGradient(start, end)` if the drag is
  non-trivial.
• Live preview overlay (white dashed line + black-outlined endpoint
  dots) drawn after `renderTo`, identity transform for pixel-perfect
  placement. Same pattern as the crop overlay; never bleeds into export.
• `ImageEditor.handleCommitGradient` shifts coords by crop origin if a
  crop is active, scales to source pixels, and renders the gradient via
  `ctx.createLinearGradient` onto a baseW × baseH canvas. SOURCE-res
  bitmap → sharp exports.
• New `OptionsBar` variant: FG + BG color pickers + a hint reminding
  start = FG, end = BG.

Removes `gradient` from `STUB_TOOLS`.

Branched from `main` (independent of #38, #39).

Test plan

• Press G — gradient variant appears in options bar with FG + BG color pickers.
• Drag across canvas — dashed preview line shows from cursor-down to cursor.
• Release — new layer appears, gradient goes from FG (start) to BG (end).
• Switch FG/BG with X — next gradient swaps.
• Crop the image first, then gradient inside the cropped view — gradient lands correctly within the crop.
• Cmd+Z undoes the gradient layer.
• Export PNG — gradient renders at source resolution (smooth, not blurry).
• Save/reload .json — gradient layer round-trips.

🤖 Generated with Claude Code