Show last updated date for locally installed plugins and themes

[iflairwebtechnologies]

Problem:
Currently, WordPress does not show when a locally installed plugin or theme was last modified.
This makes it difficult for site owners and developers to identify recent local changes such as
manual edits, deployments, or hotfixes that are not reflected in repository update data.

Proposed Solution:
Display a "Last updated" date for locally installed plugins and themes based on filesystem
modification time. This information is shown only in the admin UI and does not affect update
checks or permissions.

Implementation Details:

• Plugins: Uses the filesystem modification time of the plugin file/directory and displays the date in the Plugins list table meta row.
• Themes: Determines the most recent modification time using style.css as a primary reference, with a fallback scan of the theme directory, and exposes the value to the Themes screen UI.
• Dates are formatted using date_i18n() and include proper translator comments.

Benefits:

• Helps administrators quickly identify recently changed plugins or themes
• Useful for debugging, deployments, and staging/production sync checks
• UI-only enhancement with no impact on core update logic

Testing:

• Verified with single-file plugins and directory-based plugins
• Verified with classic and block themes
• Tested on fresh installs and modified local themes

Patch:
Attached patch implements the above changes.