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

migrations/0002_account_last_error.down.sql

@@ -0,0 +1 @@
+ALTER TABLE accounts DROP COLUMN last_error;

migrations/0002_account_last_error.up.sql

@@ -0,0 +1 @@
+ALTER TABLE accounts ADD COLUMN last_error TEXT NOT NULL DEFAULT '';