test(web): guard against single-brace i18n placeholders

[Yevanchen]

Fill what changed. Use N/A for irrelevant or maintainer-only items. See CONTRIBUTING.md for branch, CLA, generated file, and CI rules.

Summary

• Add a regression test to apps/web/tests/i18n-catalog-parity.test.ts that fails when any translation catalog string contains a single-brace placeholder (e.g. {time}) that the runtime t() cannot interpolate.

Why

• Bug: MCP advanced settings scroll failure, run log details overlap, timestamp rendered as {time} #501 reported the run-log header rendering the literal 创建于 {time}. The root cause was single-brace placeholders in the translation catalogs, while t() only substitutes {{name}}; fix(web): interpolate thread translation variables #522 fixed the strings, but nothing prevented the same defect class from coming back. The existing parity test only compares {{name}} variables across locales, so a {name} typo present in all locales (as it was pre-fix(web): interpolate thread translation variables #522) passes unnoticed.

Verification

• Commands: bun run test in apps/web (235 pass, 0 fail); bun run lint in apps/web (clean).
• Manual steps: temporarily reverted threads.created in zh-CN.json to the pre-fix(web): interpolate thread translation variables #522 single-brace form 创建于 {time} and confirmed the new test fails with the offending key/locale, then restored the file.
• Not run: N/A

Impact

• User/API/contract changes: none (test-only).
• Generated files / GraphQL / DB / lockfile: none.
• Env or config changes: none.
• Risk and rollback: negligible; revert the single commit.

Review

• Closest review areas: apps/web/tests/i18n-catalog-parity.test.ts, the t() interpolation contract in apps/web/src/shared/i18n/provider.tsx.
• Known trade-offs: a translation that intentionally wants a literal {word} would trip the guard; none exists today and escaping can be revisited if one appears.