Extension management improvements (#5552)

* feat: enhance asset management with extension categories

Co-authored-by: Copilot <copilot@github.com>

* fix: enhance extension name validation in server endpoints

* feat: display extension author in the extensions list

* fix: unify server error response format

Co-authored-by: Copilot <copilot@github.com>

* feat: add splash on installing third-party for the first time

* fix: add URL format validation, unify validation error messages

Co-authored-by: Copilot <copilot@github.com>

* fix: apply object freeze to EMPTY_AUTHOR value

Co-authored-by: Copilot <copilot@github.com>

* fix: typecheck extensionName in API requests

Co-authored-by: Copilot <copilot@github.com>

* feat: add feature flag guard to extensions endpoints

Co-authored-by: Copilot <copilot@github.com>

* fix: parse URL before checking

Co-authored-by: Copilot <copilot@github.com>

* fix: use case insensitive regex check

* fix: make debug log more useful

Co-authored-by: Copilot <copilot@github.com>

* fix: add pre-validation of URL format and protocol

Co-authored-by: Copilot <copilot@github.com>

* fix: leaner installation success toast

* fix: settings data loss when extensions are disabled

* fix: don't try to auto-focus elements that don't exist

Co-authored-by: Copilot <copilot@github.com>

* fix: set Popup.defaultResult to negative

Co-authored-by: Copilot <copilot@github.com>

* revert: restore undefined default result

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Co-authored-by: Copilot <copilot@github.com>