fix(core): clean up the displaced core after a failed upgrade restart

Windows publishes the core by moving the running binary aside, and only the
success path could delete it afterwards. A failed restart left it on disk,
and the uninstaller did not know about it either.

src-tauri/packages/windows/installer.nsi

@@ -1133,6 +1133,10 @@ Section Uninstall
     Delete "$INSTDIR\\{{this}}"
   {{/each}}
 
+  ; A failed core upgrade leaves the displaced binary behind; it is never user data.
+  Delete "$INSTDIR\verge-mihomo.old"
+  Delete "$INSTDIR\verge-mihomo-alpha.old"
+
   ; Delete app associations
   {{#each file_associations as |association| ~}}
     {{#each association.ext as |ext| ~}}

src-tauri/src/feat/core_upgrade.rs

@@ -99,6 +99,11 @@ pub async fn upgrade_core(force: bool) -> Result<CoreUpgradeReport> {
         }
         // Renaming one hard link over the other succeeds without consuming the name.
         let _ = std::fs::remove_file(&rollback);
+        // Redundant once the rollback link exists; without one it is the only copy left.
+        #[cfg(windows)]
+        if restorable {
+            let _ = std::fs::remove_file(target.with_extension("old"));
+        }
         return Err(error);
     }
     let _ = std::fs::remove_file(&rollback);