Add scratchpad dashboard widget with persistence

[multiplex55]

Motivation

• Provide a lightweight, persistent scratchpad widget for the dashboard that lets users quickly jot notes and persist them between sessions.
• Enable common scratchpad operations such as copying the full content and inserting a timestamp at the cursor position.
• Integrate with the dashboard refresh model so the widget can reload from stored content according to the shared refresh settings.

Description

• Add src/dashboard/widgets/scratchpad.rs implementing ScratchpadWidget with a multiline egui::TextEdit, debounced persistence to a JSON file (scratchpad.json by default), and error reporting.
• Add Copy and Append timestamp buttons in the widget UI, where timestamp insertion preserves/updates the text edit cursor using TextEditState.
• Respect the shared refresh schedule by using refresh_schedule/run_refresh_schedule and provide a header Refresh control; the widget reloads stored content on load unless there are unsaved edits.
• Register and export the widget in src/dashboard/widgets/mod.rs and add unit tests for format_timestamp, insert_at_char_index, and save/load round-trip storage.

Testing

• Added unit tests in the new module for timestamp formatting, character-index insertion, and save/load round-trip behavior; these tests are included under #[cfg(test)] in scratchpad.rs.
• cargo test --workspace --quiet was started during the rollout but was interrupted before completion, so a full test run did not complete.

---

Codex Task