feat(settings): rename Import button to Custom URL; bump to 4.4.8

The online update always pulls every preset Celestrak/amsat URL and
the custom URL lived hidden behind the "Import" dialog's toggle. The
user wants a discoverable entry point to edit the custom source URLs.

Rename the data card button "Import" -> "Custom URL" (en: Custom URL,
zh: 自定义URL, tr: Özel URL). The existing dialog already hosts both
URL text fields (TLE + transceivers) and the file-import buttons, so
no dialog changes are needed - the import feature stays inside, as
the user requested.

Version bump per fork convention (feature change): 4.4.7 -> 4.4.8,
versionCode 447 -> 448. v4.4.7 release stays downloadable.

What's-new dialog updated in all three locales with this change.

Verified: settings + app compile, unit tests unaffected.

gradle/libs.versions.toml

@@ -1,8 +1,8 @@
 [versions]
 #noinspection UnusedVersionCatalogEntry
-appVersionCode = "447"
+appVersionCode = "448"
 #noinspection UnusedVersionCatalogEntry
-appVersionName = "4.4.7"
+appVersionName = "4.4.8"
 #noinspection UnusedVersionCatalogEntry
 compileSdk = "37"
 #noinspection UnusedVersionCatalogEntry