US-5.3implemented Launch an interactive Dash dashboard
As a user who wants to explore my financial data interactively I want launch a local Dash web dashboard that shows both the Sankey diagram and the Treemap in a browser so that I can zoom, filter, and interact with the visualisations.
Acceptance Criteria
- Running `hledger_plot` opens a browser tab at `localhost`.
- The dashboard shows both Sankey and Treemap plots.
- The `SKIP_DASH=true` environment variable disables the dashboard (for headless/test environments).
No DAG diagram: UI presentation layer — combines US-5.1 and US-5.2 outputs in a Dash app; no new data path, just a viewer for existing visualisations.
Demo video coming soon