feat: wireframe components, ASCII export, frame shapes for diagrams (0.43.0)
Wireframe component library (#648): 16 UI components rendered as SVG
wireframes — browser window (traffic lights + URL bar), mobile frame
(notch + home indicator), modal (shadow + close X), card (image placeholder),
button, input field, select dropdown, checkbox, radio, toggle switch,
navbar, table, tabs, progress bar, avatar, alert.
Dropdown palette in toolbar with categorized sections (Frames, Form,
Layout, Display). Components placed at canvas center with appropriate
default sizes.
ASCII/Unicode text export (#649): converts whiteboard to text art using
box-drawing characters. Shapes become outlined boxes with centered labels.
Arrows render as horizontal/vertical/L-shaped lines with arrowheads.
Grid capped at 200x100 characters. Export via TXT toolbar button.
ShapeKind type extended with 16 wireframe variants. Rendering switch
updated. All shapes render in existing SVG pipeline.
Closes #648, #649, #650