feat: tabular explorer improvements

[ThibaudDauce]

Fix datagouv/data.gouv.fr#1950

• New /explore landing page with breadcrumb, hero, tabular-only dataset search and configurable "featured resources" section
• featuredResourceIds config exposed in runtimeConfig.public (env override NUXT_PUBLIC_FEATURED_RESOURCE_IDS)
• New /explore?resource_id=… resource view with custom header (filename, producer, format, download button)
• Header dropdown: searchable list of all resources of the current dataset; non-explorable ones shown but disabled
• Tabs on the resource view (Données / Structure des données / Description / Métadonnées / Téléchargements / Swagger) with URL state via ?tab=…
• Breadcrumb navigation back to search preserves the active query (?q=…)
• TabularExplorer now embedded in the dataset accordion (ResourceExplorerViewer) in place of the small static preview, so /explore and the dataset page share the same explorer
• Resource selector dropdown also added to the dataset accordion header (alongside the existing sidebar on desktop, replacing the mobile sidebar dialog)
• useHasTabularData filter now also excludes resources whose source URL last check returned >= 400, hiding broken resources from search results and pickers
• DatasetCard exposes a default slot to override the description block (used to highlight the file on /explore cards)
• Downloads block extracted from the accordion into its own reusable component, used on both /explore and the dataset accordion
• TabularExplorer columns auto-distribute to fill the available width on first render then lock — no layout shift on pagination/filtering, table fills the page
• TabularExplorer toolbar and active-filters bar constrained to the page container width while the table itself spans edge-to-edge
• Clear loading and error states in TabularExplorer when the profile or data is unavailable, replacing the silent infinite-loader case
• Tabular profile fetched once per resource and shared between TabularExplorer and DataStructure via provide/inject — no duplicate request when both tabs are active