Administration: Correct the message shown when build files are unavailable

[i-am-chitti]

Trac ticket: https://core.trac.wordpress.org/ticket/66022

The Font Library and Connectors screens tell the user to run npm install when build files are missing. That was correct when 61492 added a postinstall hook that built the assets; 62321 removed it and the messages were never updated. The two strings are also inconsistent — one says "Gutenberg build files", the other "build files".

Changes

• npm install → npm run build in both messages.
• Dropped the "Gutenberg" reference so both strings read consistently.
• Moved the command into a sprintf() placeholder with a translators comment, per src/index.php.

Screenshots

Font Library

Before	After

Connectors

Before	After

Use of AI

AI assistance: Yes
Tool(s): Claude Code
Model(s): Sonnet 5
Used for: Go through changesets

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