fix(QRCode): guard against a non-positive size prop

[KayProject]

Resolves #310

Scope note

Two of the three items in the issue have already landed on main since it was filed:

• Canvas clearing / ghost artifacts — the hand-rolled fillRect grid loop the clearRect fix targeted is gone. QRCode now delegates to QRCodeLib.toCanvas, which sets the canvas dimensions and repaints the full area on each draw, so there's no partial-overdraw window left to fix.
• figure / figcaption — already in place; the wrapper is a <figure> and the label renders in a <figcaption>.

That leaves the size guard, which this PR implements.

The fix

size defaulted to 160 but was never validated. size={0} or a negative value gave the canvas invalid dimensions and rendered as a broken box with no indication of why.

Now a non-positive size logs [QRCode] size must be > 0 and skips the draw entirely — getContext is never reached.

The check is derived at render rather than stored in state, for two reasons:

1. Setting state inside the effect trips the repo's react-hooks/set-state-in-effect rule.
2. Returning early from the effect without settling isLoading would leave the spinner running forever, waiting on a draw that is never attempted. Deriving it lets the render skip the spinner directly.

Verification

npx vitest run src/components/QRCode.test.tsx — 15 passed, including 4 new cases: warn + no draw for size={0}, same for a negative size, no spinner left running for an invalid size, and no false warning for a valid size.

ESLint clean on both changed files.

Heads-up on the build AC

npm run build doesn't pass on main for unrelated reasons — FeeEstimator.tsx and SorobanPanel.tsx each carry an unclosed block from a merge (also the source of 61 pre-existing test failures). Happy to open a separate repair PR.

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