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/store/account_errors.go

@@ -0,0 +1,61 @@
+package store
+
+import (
+	"context"
+	"time"
+)
+
+// AccountError is one concrete error recorded during an account's run —
+// folder-level, message-level, or account-level (connect/login).
+type AccountError struct {
+	ID         int64     `json:"id"`
+	AccountID  int64     `json:"account_id"`
+	RunID      int64     `json:"run_id"`
+	Kind       string    `json:"kind"` // folder | message | account
+	Folder     string    `json:"folder"`
+	MessageRef string    `json:"message_ref"`
+	Error      string    `json:"error"`
+	CreatedAt  time.Time `json:"created_at"`
+}
+
+// AddAccountError appends one error row for an account's current run.
+func (s *Store) AddAccountError(ctx context.Context, accountID, runID int64, kind, folder, ref, msg string) error {
+	_, err := s.Pool.Exec(ctx,
+		`INSERT INTO account_errors (account_id, run_id, kind, folder, message_ref, error)
+		 VALUES ($1,$2,$3,$4,$5,$6)`,
+		accountID, runID, kind, folder, ref, msg)
+	return err
+}
+
+// ClearAccountErrors removes an account's errors at the start of a run, so the
+// list reflects only the current run (mirrors ResetAccountCounters).
+func (s *Store) ClearAccountErrors(ctx context.Context, accountID int64) error {
+	_, err := s.Pool.Exec(ctx, `DELETE FROM account_errors WHERE account_id=$1`, accountID)
+	return err
+}
+
+// ListAccountErrors returns an account's errors in insertion order, for the
+// per-account error modal.
+func (s *Store) ListAccountErrors(ctx context.Context, accountID int64) ([]AccountError, error) {
+	rows, err := s.Pool.Query(ctx,
+		`SELECT id, account_id, run_id, kind, folder, message_ref, error, created_at
+		 FROM account_errors WHERE account_id=$1 ORDER BY id`, accountID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []AccountError{}
+	for rows.Next() {
+		var e AccountError
+		var runID *int64
+		if err := rows.Scan(&e.ID, &e.AccountID, &runID, &e.Kind, &e.Folder,
+			&e.MessageRef, &e.Error, &e.CreatedAt); err != nil {
+			return nil, err
+		}
+		if runID != nil {
+			e.RunID = *runID
+		}
+		out = append(out, e)
+	}
+	return out, rows.Err()
+}