Create per-dataset hidden dashboard pages with stable nav links

[JZauner]

Motivation

• Replace the single dashboard page with a dedicated (hidden) nav page per dataset so each dataset has its own stable dashboard instance while selection remains driven by the dataset manager.
• Ensure renames keep dashboard links stable and deletes remove the corresponding dashboard page.

Description

• Add R/dataset_nav_pages.R with new_dataset_page() to create stable nav_value/module_id pairs and a mutable dataset_name handle, and reconcile_dataset_pages() to compute added/removed/renamed datasets.
• Rework the app server in R/LightLogWeb-app.R to dynamically nav_insert() a hidden nav_panel per dataset, datasetDashboardServer() instantiation per page, and nav_remove() on deletion; navigation is driven by nav_select() using the selected dataset name.
• Simplify datasetDashboardServer() signature in R/m_dataset_dashboard.R by removing active_panel coupling so the dashboard module can be instantiated per dataset page without changing its internal UI/content.
• Add test scaffolding and unit tests in tests/testthat/ for the new page helpers, and add testthat to DESCRIPTION suggests for CI.

Testing

• Added unit tests under tests/testthat/test-dataset-nav-pages.R covering new_dataset_page() and reconcile_dataset_pages() semantics and a test for rename detection, and a tests/testthat.R test runner was added.
• Attempted to run tests with Rscript -e "testthat::test_dir('tests/testthat')" but execution failed in this environment because Rscript is not available; tests could not be executed here.

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Codex Task