fix: consistent error reporting + reset mapping modal per account

Error consistency: folder-level failures were counted only toward the task's
done_with_errors status, not the account's error_count, so a task showed
DONE_WITH_ERRORS while its only account showed 0 errors / DONE. Now folder
errors increment the account counter and the account status becomes
done_with_errors when errs>0.

Visibility: persist accounts.last_error (migration 0002) so the failing folder
/ login error survives a page reload (shown red under the source login);
cleared at the start of each run.

Modal reset: the folder-mapping modal kept its selections across opens, so
adding a second account showed the first account's mapping. It now mounts
fresh per add (conditional render + key), reflecting the newly-probed folders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/httpapi/accounts.go

@@ -21,6 +21,7 @@ type AccountView struct {
 	Copied        int64  `json:"copied"`
 	Skipped       int64  `json:"skipped"`
 	Errors        int64  `json:"errors"`
+	LastError     string `json:"last_error,omitempty"`
 }
 
 func accountDTO(a store.Account) AccountView {
@@ -28,6 +29,7 @@ func accountDTO(a store.Account) AccountView {
 		ID: a.ID, SrcLogin: a.SrcLogin, DstLogin: a.DstLogin,
 		TestSrcStatus: a.TestSrcStatus, TestDstStatus: a.TestDstStatus,
 		Status: a.Status, Copied: a.Copied, Skipped: a.Skipped, Errors: a.Errors,
+		LastError: a.LastError,
 	}
 }