Discard all: clear the canvas, keep the diagram (FL-96)

[raj-khan]

FL-96, from feedback: a Discard all option.

• Discard all, in the menu (shown in red, disabled when there is nothing) and the command palette, removes every shape, stroke and connection in one change, as Excalidraw's Reset the canvas does.
• The title, diagram notes, style and the open file are kept, so it is a clean sheet of the same diagram; New diagram is still the way to start a different one.
• No confirmation, as with New diagram: the toast says how many shapes went, and Undo brings them all back.

Tests: 341 unit, 159 e2e, all passing.

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