Allow importing account-less legacy instances as sources

Legacy v1 instances without an account had no import path after the
switch to explicit account review. Surface them in a separate "Sources"
section of the legacy import view where they can be added with a single
tap (no sign-in) or removed, mirroring the accounts flow. Instances tied
to an account stay in the Accounts section and are not duplicated.

Detection now gates the entry points and first-launch prompt on any
legacy data (accounts or sources), not accounts alone.

Yattee/Localizable.xcstrings

@@ -5828,6 +5828,46 @@
         }
       }
     },
+    "migration.sources.footer" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Add these sources from the previous Yattee app. No sign-in is required."
+          }
+        }
+      }
+    },
+    "migration.sources.imported.title" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Source Added"
+          }
+        }
+      }
+    },
+    "migration.sources.remove.title" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Remove Legacy Source?"
+          }
+        }
+      }
+    },
+    "migration.sources.section" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Sources"
+          }
+        }
+      }
+    },
     "migration.subtitle" : {
       "extractionState" : "stale",
       "localizations" : {