fix(theme-common): report recoverable storage errors via reportError() (AI-assisted)

[LeSingh1]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Closes #6747.

Today, when Docusaurus's createStorageSlot (used for color mode, announcement bar dismissal, etc.) hits a failing localStorage/sessionStorage operation — for example in an iframe, an incognito session, or under strict browser privacy settings — it logs to console.error and continues. Site owners using window.onerror, window.addEventListener('error', ...), or third-party error reporting may not see these failures today, even though they can be useful diagnostics for real browser/runtime conditions.

The HTML spec's reportError() API (Baseline since 2022) is purpose-built for this: it dispatches an ErrorEvent to global listeners just like an uncaught exception would, without actually throwing. This makes the error observable to host-page tooling while keeping the recovery path unchanged.

What this PR does

• Adds a tiny reportRecoverableError(error) helper in @docusaurus/theme-common that prefers globalThis.reportError and falls back to console.error when it's unavailable (older browsers, some test runners).
• Migrates the four console.error call sites inside createStorageSlot (get / set / del / listen) to use the helper. Original messages are preserved as the new Error's message; the underlying browser exception is preserved via Error.cause (a pattern already used elsewhere in the repo — see vitest.config.ts's snapshotErrorCause.ts serializer).
• Leaves intentional console.error calls untouched: deprecation warnings in colorMode.tsx, locale diagnostics in usePluralForm.ts, and CLI/server logging. Those are not recoverable runtime errors.

Scope was kept deliberately small so the helper and approach can be reviewed in isolation; follow-up PRs can migrate additional call sites incrementally if maintainers are happy with the pattern.

Test plan

• yarn test packages/docusaurus-theme-common/src/utils/__tests__/errorUtils.test.ts — 2/2 pass, covers both the reportError-present and fallback paths.
• yarn test packages/docusaurus-theme-common — 108/108 pass, no regressions in existing storage / color mode behavior.
• yarn tsc --noEmit in packages/docusaurus-theme-common — clean.
• yarn lint:js — no new errors or warnings on the changed files.

Notes

This change is AI-assisted, per AGENTS.md. The fallback to console.error preserves the existing behavior for older browsers and test environments without reportError.

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	2c56c23
🔍 Latest deploy log	https://app.netlify.com/projects/docusaurus-2/deploys/6a060efe6e091a0009c9588d
😎 Deploy Preview	https://deploy-preview-12021--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.