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_test.go

@@ -0,0 +1,75 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+// AddAccountError persists individual errors; ListAccountErrors returns them in
+// insertion order; ClearAccountErrors wipes them for the next run.
+func TestAccountErrorsAddListClear(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+	epSrc, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "src", Host: "a", Port: 993, TLSMode: "ssl"})
+	epDst, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "dst", Host: "b", Port: 993, TLSMode: "ssl"})
+	taskID, _ := s.CreateTask(ctx, Task{Name: "t", SrcEndpointID: epSrc, DstEndpointID: epDst})
+	runID, _ := s.CreateRun(ctx, taskID, "manual")
+	accID, _ := s.CreateAccount(ctx, Account{TaskID: taskID, SrcLogin: "u", SrcPassEnc: "x", DstLogin: "u2", DstPassEnc: "y"})
+
+	if err := s.AddAccountError(ctx, accID, runID, "folder", "INBOX", "", "examine failed"); err != nil {
+		t.Fatalf("add folder: %v", err)
+	}
+	if err := s.AddAccountError(ctx, accID, runID, "message", "INBOX", "UID 42: hi", "append rejected"); err != nil {
+		t.Fatalf("add message: %v", err)
+	}
+
+	errs, err := s.ListAccountErrors(ctx, accID)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(errs) != 2 {
+		t.Fatalf("len=%d want 2", len(errs))
+	}
+	if errs[0].Kind != "folder" || errs[0].Folder != "INBOX" || errs[0].Error != "examine failed" {
+		t.Fatalf("errs[0]=%+v", errs[0])
+	}
+	if errs[1].Kind != "message" || errs[1].MessageRef != "UID 42: hi" || errs[1].Error != "append rejected" {
+		t.Fatalf("errs[1]=%+v", errs[1])
+	}
+	if errs[0].ID >= errs[1].ID {
+		t.Fatalf("expected insertion order by id: %d then %d", errs[0].ID, errs[1].ID)
+	}
+
+	if err := s.ClearAccountErrors(ctx, accID); err != nil {
+		t.Fatalf("clear: %v", err)
+	}
+	errs, err = s.ListAccountErrors(ctx, accID)
+	if err != nil {
+		t.Fatalf("list after clear: %v", err)
+	}
+	if len(errs) != 0 {
+		t.Fatalf("after clear len=%d want 0", len(errs))
+	}
+}
+
+// Deleting an account cascades its errors (ON DELETE CASCADE).
+func TestAccountErrorsCascadeOnAccountDelete(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+	epSrc, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "src", Host: "a", Port: 993, TLSMode: "ssl"})
+	epDst, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "dst", Host: "b", Port: 993, TLSMode: "ssl"})
+	taskID, _ := s.CreateTask(ctx, Task{Name: "t", SrcEndpointID: epSrc, DstEndpointID: epDst})
+	accID, _ := s.CreateAccount(ctx, Account{TaskID: taskID, SrcLogin: "u", SrcPassEnc: "x", DstLogin: "u2", DstPassEnc: "y"})
+	_ = s.AddAccountError(ctx, accID, 0, "account", "", "", "login failed")
+
+	if err := s.DeleteAccount(ctx, accID); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+	var n int
+	if err := s.Pool.QueryRow(ctx, `SELECT count(*) FROM account_errors WHERE account_id=$1`, accID).Scan(&n); err != nil {
+		t.Fatalf("count: %v", err)
+	}
+	if n != 0 {
+		t.Fatalf("account_errors not cascaded: %d rows", n)
+	}
+}