feat(errors): per-account error modal with persisted error list

Accounts finishing done_with_errors showed only a count and a single
last_error. This adds a modal listing every concrete error of the
account's most recent run.

- migration 0005: account_errors table (kind folder|message|account,
  folder, message_ref, error, created_at; ON DELETE CASCADE; indexed)
- store: AddAccountError / ClearAccountErrors / ListAccountErrors
- copy: OnError callback captures per-message error text (previously
  only counted), with a "UID N: subject" reference
- orchestrator: clear errors at run start; persist folder/message/
  account errors; cap 500 rows/account/run with a suppressed-note row
- api: GET /api/tasks/{id}/accounts/{accountId}/errors
- web: AccountErrorsModal, clickable ERRORS count, api + styles

Verified: migration applies on Postgres 18; store add/list/clear and
cascade tests pass against real pg; backend build/vet/test green; web
tsc+vite build and oxlint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9Eq4JtWjyNTv5qat3B3mM

internal/httpapi/router.go

@@ -24,6 +24,7 @@ func (s *Server) Router() http.Handler {
 	api.HandleFunc("PUT /api/tasks/{id}/folder-mapping", s.handleSetFolderMapping)
 	api.HandleFunc("PUT /api/tasks/{id}/schedule", s.handleSetSchedule)
 	api.HandleFunc("GET /api/tasks/{id}/runs", s.handleListRuns)
+	api.HandleFunc("GET /api/tasks/{id}/accounts/{accountId}/errors", s.handleListAccountErrors)
 	api.HandleFunc("DELETE /api/tasks/{id}/accounts/{accountId}", s.handleDeleteAccount)
 	api.HandleFunc("POST /api/tasks/{id}/import", s.handleImportCSV)
 	api.HandleFunc("POST /api/tasks/{id}/test", s.handleTestAccounts)