Customize: Don't let hardcoded regex override customize_allowed_urls filter

[abhi3315]

Trac ticket: https://core.trac.wordpress.org/ticket/65030

The hardcoded regex in isLinkPreviewable() and the previewUrl setter blocks any URL with /wp-content/ in the path, even when the URL was explicitly allowed via customize_allowed_urls. This breaks the Customizer for sites installed under a /wp-content/ subdirectory.

Fix: capture the matched allowed URL's path prefix, apply the regex only to the remainder beyond it.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.6
Used for: Analysis of the bug and implementation approach. Final code implemented, reviewed, and verified by me.

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