refactor(blog): dedupe the 'string-or-JSON.stringify' block field shape

[pedrofrxncx]

Source: reduction found while auditing apps/web/src/components/sandbox/content/blog/ (blog sandbox UI area) for dead code/duplication, per the ship-it checklist's dedup guidance.

What: BlockEditor in blocks/block-registry.tsx repeated the identical ternary — "if this block field is already a JSON string, pass it through, otherwise JSON.stringify it (defaulting to [])" — three times (CardGroup's cards, Table's headers, Table's rows). Extracted the one-line jsonStr() helper (matching the existing local str() helper's style/placement in this file) and replaced all three call sites.

Why a maintainer wants it: removes copy-pasted logic that would otherwise need three edits if the field-normalization rule ever changes (e.g. a null vs [] default), with zero behavior change.

Net delta: -15 / +8 lines, purely mechanical — same runtime behavior (verified by reading: jsonStr(v) is byte-identical to each removed ternary).

Reviewer check: cd apps/web && bunx tsc --noEmit -p . (no new errors from this file) and bunx oxlint src/components/sandbox/content/blog/blocks/block-registry.tsx (0 warnings/errors).

Locally verified: bun run fmt, bunx oxlint on the changed file, and a scoped tsc --noEmit in apps/web (no errors touching this file). No existing test covers this file's render branches, so nothing to invert; full CI validates the rest.

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Summary by cubic

Extracts the repeated string-or-JSON.stringify block field normalization logic in BlockEditor into a single jsonStr() helper, removing three copies of the same ternary. Behavior is unchanged; future changes to the default (e.g., null vs []) become a one-line edit.

^{Written for commit 55c49ee. Summary will update on new commits.}

[pedrofrxncx]

Closing as stale: this PR sat past the bot's 48h merge window, main has moved on, and its CI results no longer reflect the current base. This is a housekeeping close, not a rejection of the change — if the underlying problem still exists, the bot will find it again and open a fresh, rebased PR.

[studio-bot:stale-close]