fix(editor): make plugin panel dev detection webpack-safe

[smokie40]

BuildBook's authoritative external next build --webpack Gate 9 composition check is green, but its log still reports a Webpack critical-dependency warning from plugin-panels.ts because isDevMode() assigns bare import.meta to a local variable.

Webpack explicitly supports property access or destructuring of import.meta. This patch keeps the existing dev-mode fallback semantics but destructures only env, avoiding the unsupported bare import.meta access.

This intentionally does not expand @pascal-app/editor/embed with the full block-selection UI stack merely to hide the warning. Kind-owned editing chunks may continue to consume the normal editor API; the browser-safe embed boundary remains narrow.

Acceptance:

• full Pascal CI must remain green;
• BuildBook will pin the exact merge SHA and rerun the real ViewerStage + external shadow scene next build --webpack gate;
• the final BuildBook log must no longer contain the plugin-panels.ts direct import.meta warning.