Adding more robust version of the app

[Amorfati123]

Power / "fetch anything, do anything":

• Data browser now covers indicators, data elements, program indicators, and data sets (reporting rates), with server-side search so it scales to the whole instance instead of downloading everything.
• Organisation unit tree with lazy-loaded children and multi-select.
• Flexible periods: relative presets or custom fixed periods (e.g. 2021;2022;2023), plus an include disaggregations option.
• Advanced raw-API loader: fetch any DHIS2 API endpoint as JSON into an R variable, for data beyond analytics.
• Editor gained auto-indent, bracket completion, save/open .R files, and richer examples (forecasting, outlier detection, correlation, trend).

Robustness:

• Analytics batching with bad-item isolation..
• Timeouts and retries on all network calls.
• Console truncation, busy spinner, graceful error hints, toast notifications.

Professional / aesthetic:

• Resizable four-pane workspace (draggable splitters, sizes remembered).
• Light and dark themes.
• Refreshed design system, History tab, environment View/CSV/Remove.