fix(admin): keep JSON snippets readable after save

[poboll]

Summary

• format JSON snippets with two-space indentation when they are saved
• add regression coverage for readable output and invalid JSON handling

Root cause

The snippet editor used JSON.stringify(JSON.parse(raw)) during save. That validation step also collapsed valid JSON into a single line, so reopening the snippet lost the readable structure shown in the editor. The structured snippet conversion path already uses two-space indentation, making the two save paths inconsistent.

Scope

This addresses the JSON snippet formatting regression reported in #2749. The current Admin writing flow has since been substantially revised, so this PR does not claim to close the separate preview and draft-state reports from that issue.

Verification

• pnpm -C apps/admin exec vitest run src/features/snippets/utils/snippets.normalize.test.ts (2 tests passed)
• pnpm -C apps/admin run typecheck
• pnpm -C apps/admin run build
• ESLint on the changed TypeScript files
• git diff --check

Related to #2749

[safedep]

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