feat(design): keyboard path for free artboard placement and the minimap in the canvas #3204

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Problem or opportunity

Follow-up from @egdev6's review of #3146 (Gentle Design canvas, layout and minimap; part of #2740).

In the design canvas, moving an artboard to a free position is pointer-only: you drag it. The minimap (role="img") is pointer-only too: you click or drag in it to move the view. Keyboard users can use Row/Column/Grid, pan with the arrow keys, zoom with +/- and fit with 0, so nothing is blocked. Free placement itself, though, has no keyboard path, which WCAG 2.1.1 (Keyboard) and 2.5.7 (Dragging Movements) ask for. The docs now say both features are pointer-only.

Proposed outcome

• Artboards: a selected artboard can be moved with the keyboard, for example with Alt or a modifier plus the arrow keys, in steps of a few canvas pixels and larger steps with Shift. The layout becomes Free and is saved once when the key is released, the same as after a drag. The move is announced to assistive technology.
• Minimap: it becomes focusable, with an accessible name. While it has focus, the arrow keys move the view, or there is a documented keyboard alternative it points to.
• The Keyboard shortcuts list and docs/gentle-shell.md describe both.

Alternatives considered

• Keep Row/Column/Grid as the only keyboard path. This is what ships today, but it gives keyboard users no free placement.
• A "Move to…" dialog with x/y fields. It is precise, but slower than arrow-key nudging for small adjustments.

Additional context

• Review: feat(design-canvas): layout.json arrangements, moving artboards and the minimap #3146 (review by @egdev6, marked non-blocking for that PR).
• Related keyboard work: feat(design): navigate between artboards with the keyboard in focus mode #2685 (navigating between artboards in focus mode) and feat(design): Figma-style canvas shortcuts: Space-drag pan, double-click to zoom an artboard, Shift+1/2 #2830 (canvas shortcuts).