Remove dead legacy-import pipeline and unused LegacyAccount.password

The manual account-import flow made the old silent instance-import
pipeline unreachable. Drop the orphaned parsing, reachability checks,
batch import, result/error types, and the old import row view, plus the
httpClient dependency they were the only consumer of. Also remove the
LegacyAccount.password field, which was parsed but never read.

Yattee/Core/AppEnvironment.swift

@@ -296,8 +296,7 @@ final class AppEnvironment {
             invidiousCredentialsManager: invidiousCreds,
             pipedCredentialsManager: pipedCreds,
             invidiousAPI: invidiousAPI,
-            pipedAPI: pipedAPI,
-            httpClient: client
+            pipedAPI: pipedAPI
         )
 
         // Initialize Sources Settings