fix(theme): restore copy-text-to-clipboard as lazy fallback for non-secure contexts

[dmoranp]

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 CopyButton throws "Cannot read properties of undefined (reading 'writeText')" in non-HTTPS environments after 3.9.x #11791).

Motivation

PR #11422 removed copy-text-to-clipboard and replaced it with navigator.clipboard, assuming broad HTTPS adoption. However, navigator.clipboard is only available in secure contexts — it is undefined in non-HTTPS environments (e.g. http://192.168.x.x:3000 on a local network), causing:

TypeError: Cannot read properties of undefined (reading 'writeText')

This PR re-introduces copy-text-to-clipboard as a lazily loaded fallback: it is only imported via dynamic import() when navigator.clipboard is unavailable, so HTTPS users pay no bundle cost.

Behavior:

• HTTPS / localhost: uses navigator.clipboard.writeText() as before
• non-HTTPS: lazily loads copy-text-to-clipboard and uses it as fallback
• Errors: intentionally not caught, so they remain unhandled and can be captured by observability tools (Sentry, PostHog, etc.)

Closes #11791

Test Plan

To reproduce the bug (before fix):

1. npm run start -- --host 0.0.0.0
2. Access via http://192.168.x.x:3000 from another device
3. Click copy button on any code block → crash

After fix: copy button works on both HTTP and HTTPS.

Related issues/PRs

• Closes CopyButton throws "Cannot read properties of undefined (reading 'writeText')" in non-HTTPS environments after 3.9.x #11791
• Caused by fix(theme): fix copy of indented code blocks, replace copy-text-to-clipboard by clipboard API #11422
• Related: fix(theme): add clipboard fallback for non-HTTPS environments #11792

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[slorber]

LGTM thanks

I created a simpler abstraction and added the missing lockfile change