a11y: Add accessible labels to canvas tool-rail buttons

Summary Icon-only buttons in the tool rail likely lack accessible names, making them unreadable to screen readers. Add aria-label (or title) to each. (Suggest adding an accessibility label to this issue.)

Where

• apps/studio/src/features/studio (the canvas tool rail) and shared UI in apps/studio/src/shared/ui.

Steps

1. Branch: git checkout -b a11y/toolrail-aria-labels.
2. Find the icon buttons in the tool rail (bounding box, polygon, keypoint, etc.) — <button> elements without text children.
3. Add a descriptive aria-label to each (e.g., aria-label="Bounding box tool").
4. Verify with the browser devtools accessibility panel or yarn studio.
5. Run yarn lint and yarn typecheck; commit fix(a11y): add aria-labels to tool rail buttons and open a PR.

Acceptance criteria

• Every icon-only tool button has a meaningful accessible name.
• No new lint/type errors.

Good first issue — comment here if you'd like to take it.