Feat(canvas): Replace Rectangle tool with new Shapes tool

[DustyShoe]

Summary

This PR replaces the existing Rectangle tool with a new Shapes tool in the canvas toolbar.

It introduces a single Shapes tool with multiple modes in the top toolbar:

• Rect
• Oval
• Polygon
• Freehand

The tool supports live preview while drawing, uses the active fill color including alpha, and supports subtractive drawing with Ctrl / Cmd, including a dedicated subtract cursor. Rect and oval support modifier-based drawing behavior, polygon supports start-point close/highlight behavior and snapping, and drawing continues correctly even when the pointer leaves the canvas.
(New) While drawing rects and ovals, holding Space now temporarily switches the interaction from resizing to moving the uncommitted shape, without canceling the draw.

There is some intentional overlap with Lasso, because Shapes can also be used on an active Inpaint Mask layer. That said, Lasso still provides broader mask-oriented behavior: it can create a new mask layer automatically when none exists, and it remains a distinct tool with its own hotkey and dedicated masking workflow. In that sense, the overlap is acceptable: Shapes adds general-purpose filled-shape drawing across raster targets, while Lasso remains the more specialized mask-construction tool.

canvasSettingsSlice and canvasSlice are both covered by existing schema-based persistence migrations. The new shapeType field has a default value, and the canvas object schema change is additive/backward-compatible, so no separate manual migration was needed.

QA Instructions

• Open the canvas and select the Shapes tool. Confirm the old Rectangle button is replaced by a Shapes button, and the top toolbar shows Rect, Oval, Polygon, and Freehand.
• UI sanity check: top toolbar icons should render at a consistent size, and the custom gradient icons should not look undersized next to the other toolbar icons.
• On a raster layer, set the active color to a semi-transparent value and draw each shape type. Confirm the committed fill uses the active color and respects its alpha.
• Hold Ctrl / Cmd while drawing each shape type. Confirm the operation becomes subtractive, fully clears pixels regardless of active color alpha, and shows the minus-badge crosshair cursor.
• Rect: drag from corner to corner, confirm live preview and commit on pointer release. (New) Hold Shift to constrain to a square. Hold Alt to draw from center. Start the drag inside the canvas, move outside the canvas bounds, and confirm preview/commit still works. Press Esc mid-draw and confirm cancel.
• (Add): While still holding the mouse button during an active draw, hold Space and confirm the uncommitted shape can be moved instead of resized. Release Space and confirm resizing can continue.
• Oval: verify live preview and commit on release. Hold Shift to constrain to a circle. Hold Alt to draw from center. Also verify outside-canvas drag continuation and Esc cancel.
• (Add): While still holding the mouse button during an active draw, hold Space and confirm the uncommitted shape can be moved instead of resized. Release Space and confirm resizing can continue.
• Polygon: click to place vertices, confirm the pending edge previews to the current pointer position, and confirm the start vertex grows/highlights when the pointer is close enough to close the shape. Click the start vertex to close and commit. Hold Shift and verify snapping to horizontal, vertical, and 45-degree directions. Press Esc to cancel.
• (Fix): During an active polygon session, hold Space and confirm panning starts immediately without an extra click, and the unfinished polygon remains visible while panning.
• Freehand: click-drag-release to create a filled freehand contour. Confirm live preview while dragging, correct commit on release, and correct behavior if release happens outside the canvas.
• On subtractive polygon and freehand shapes, confirm no visible contour/outline remains after commit.
• Repeat core Shapes flows on an active Inpaint Mask layer. Confirm Shapes draws into the currently active mask layer.

Merge Plan

Would be best if we merge #9117 before this one.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[ufuksarp]

I only did a quick test, but it works great and feels really nice to use. While I can't comment on the coding implementation since that's not really my area, from a user perspective it's intuitive and smooth.

I'd really like to see these 2 improvements:

• Allow cycling through the shape modes when you repeatedly press the hotkey (U).
• Make holding Shift turn rectangles into perfect squares, like how it works with Oval.

Really appreciate it.

[DustyShoe]

@ufuksarp Thank you for trying this PR and giving a feedback.

• Make holding Shift turn rectangles into perfect squares, like how it works with Oval.

Seems reasonable. I'll add that.

• Allow cycling through the shape modes when you repeatedly press the hotkey (U).

Not sure how the hotkeys are implemented and if it is possible to cycle through modes with hotkey.
Also not sure if it is actually a desirable behavior. Would like to get more opinions on this one.

[DustyShoe]

I think if we’re going to implement tool hints from #9117, it would be best to merge it before this one.

This way I can add the hints directly without creating an additional PR.

[dunkeroni]

I fixed a minor weirdness around hitting Escape when also using a hotkey to pan/color-pick while drawing a shape.

Otherwise, looks good and has a lot of helpful capabilities. Good work on this.

[lstein]

Happy to see this get over the finish line!