Add an architecture and local data-flow guide

## User problem New contributors can see the file list but do not have one concise explanation of how a Windows sample becomes a database period, dashboard metric, and offline HTML report. This increases the risk of changing the wrong layer or bypassing a privacy boundary. ## Expected result Create a short contributor-focused architecture guide and link it from the README and contributing guide. ## Likely files - a new `docs/ARCHITECTURE.md` - `README.md` - `CONTRIBUTING.md` ## Acceptance criteria - Describe the flow from `WindowsActivityProvider` to `ActivityTracker`, `ActivityDatabase`, reporting, categories, and analytics. - Explain the UI worker thread and message queue at a high level. - Identify which module owns sampling, transitions, persistence, clipping, categorization, and rendering. - Document local storage paths for source and installed execution. - Highlight privacy boundaries: no network, titles may be sensitive, and tests must use synthetic data. - Include a compact text or Mermaid diagram that renders correctly on GitHub. - Keep the guide implementation-oriented and under roughly 1,200 words. ## Validation ```powershell python -m unittest discover -v git diff --check ```