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

migrations/0005_account_errors.down.sql

@@ -0,0 +1 @@
+DROP TABLE account_errors;

migrations/0005_account_errors.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE account_errors (
+    id          BIGSERIAL PRIMARY KEY,
+    account_id  BIGINT NOT NULL REFERENCES accounts(id) ON DELETE CASCADE,
+    run_id      BIGINT,
+    kind        TEXT NOT NULL CHECK (kind IN ('folder','message','account')),
+    folder      TEXT NOT NULL DEFAULT '',
+    message_ref TEXT NOT NULL DEFAULT '',
+    error       TEXT NOT NULL,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX account_errors_account_id_idx ON account_errors (account_id);