feat(dashboards): persist a default dataset for new tiles

[VibeSWE]

Description

Dashboards previously selected datasets only per tile. Add a saved Default dataset to dashboard creation and editing so new tiles start with the dashboard's chosen dataset after reopening it. Existing tile queries and dataset overrides remain unchanged; PromQL defaults require a compatible metrics dataset.

Persist the selection in server documents and list summaries, show it in the dashboard list and header, and retain it through duplication, JSON import/export, SQL chart creation, and legacy local migration. Existing dashboards remain valid without a default, and unsupported legacy dataset values do not prevent loading a dashboard.

Validation

• Frontend production build and TypeScript check passed.
• 674 dashboard-related unit tests and 2 Rust persistence/compatibility tests passed.
• 58 browser scenarios passed, including create/reload/change/clear, unavailable datasets, query-language switching, and SQL-to-dashboard creation. Two scenarios failed during the broader run and passed in an isolated rerun.
• Formatting and diff checks passed.

This PR has:

• Added comments explaining compatibility and default-dataset behavior.
• Updated dashboard API documentation.
• Tested dashboard query and persistence behavior with unit tests and mocked HTTP browser tests. Live ingestion and a live server restart were not exercised.

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[comment-clanker]

Reviewed commit 7710ada15fe0bc44a39556c5803972a510ea7a3e with GPT-6.1-Sol and Claude Opus 5.5. Both reviews found no actionable issues.

Checked saved dataset creation/editing, clearing and legacy compatibility; server persistence and summaries; defaults for new SQL and PromQL tiles; preservation of existing tile queries; and dataset retention through duplication, import/export, and SQL chart creation. Prior review concerns about legacy deserialization, export retention, and switching back from PromQL have been addressed.

Validation reported by the implementation run: production frontend build and TypeScript passed; 674 dashboard-related unit tests and 2 Rust persistence/compatibility tests passed; 58 unique browser scenarios passed, with two broader-run failures passing on rerun. Live ingestion and a live server restart were not exercised. The review agents inspected code and did not rerun these tests.

CI is still running. Merge remains conditional on green checks and no new review findings.