Add searchable dataset pickers to dashboards

[VibeSWE]

Description

Dashboard defaults previously required typing a dataset name without a way to browse existing datasets. This adds a searchable dropdown to dashboard defaults, dataset-variable defaults, and tile dataset selection. Opening the picker lists available datasets; typing ranks exact, prefix, substring, abbreviation, and small-typo matches. For example, aplgs finds api_logs and application_logs.

• Shared accessible combobox with mouse/touch selection, arrow-key navigation, Enter to select, and Escape to close the dropdown before the editor.
• Search text is separate from the saved value: browsing or typing does not dirty the dashboard, change a tile binding, or trigger schema requests. Selection requires an existing option.
• Loading, empty, no-match, retry, clear-selection, and unavailable saved-value states are handled. Large lists render at most 100 matches with a prompt to refine the search.
• Tile selection retains dataset variables and PromQL's metrics-only discovery. Changing the dashboard default affects new tiles; existing tile queries retain their datasets.

Screenshots and demo

Captured in the demo workspace using sample data.

Browse existing datasets

Fuzzy search — aplgs finds matching dataset names.

New tiles inherit the selected default

Watch the 18-second MP4 demo: change an existing dashboard default using fuzzy search, keyboard selection, Apply, and Save · Video file

Validation

Review follow-up: Opus 5.5 reproduced a false unavailable label for saved ${source} dataset-variable bindings with a failing browser regression, then fixed it. Sol 6.1 independently approved the fix; missing references still receive unavailable labels. See the resolved review thread.

• Production TypeScript/Vite build passed.
• Unit suite: 2,405 passed, 14 skipped.
• Dashboard Playwright suites (dashboards, dashboard-editor, dashboard-layout-sections, dashboard-builder-regressions): 80 passed.
• Built Storybook plus picker, dashboard-builder, and dashboard-editing browser suites: 24 passed.
• Changed-file formatting and git diff --check passed.
• Manual desktop and 390px viewport checks completed.

Browser tests use mocks and the demo workspace. Live ingestion/query validation was not run; the live dashboard test selectors were updated for the new combobox.

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This PR has:

• added comments explaining non-obvious selection and saved-value behavior.
• added documentation for the modified dataset selection behavior.

[comment-clanker]

Review found one dataset-variable display regression, reproduced with a failing browser test. Sol 6.1's component and fuzzy-search review found no blockers; Opus 5.5 is validating the integration fix.