refactor(dashboard): extract shared ResourceTable

[Shine-neko]

The listing pages each reimplement the same shell: page header with refresh marker, load-and-poll lifecycle, error banner, empty state, table skeleton. src/lib/ held no components at all, so every page carried its own copy.

This adds ResourceTable.svelte and migrates the providers page onto it as the first consumer.

Changes

• New $lib/ResourceTable.svelte: header (optional Refresh + "updated …"), load/poll lifecycle, error and empty states, table skeleton. Callers pass columns and a row snippet, plus an optional beforeTable snippet for anything page-specific between header and table (providers uses it for its stat cards).
• Providers migrated: 276 lines down to ~140 of page-specific code.

No visual change

The component carries the primitives (.page-header, .card, .alert, .empty, .btn-secondary, …) because this dashboard keeps them in each page's <style> rather than in app.css.

Verified by diffing the compiled CSS before and after, comparing the page scope and the component scope together: zero declarations lost or altered for the providers page.

Notes

Cell rules stay with the page — the row snippet is compiled into the caller, so its <td>s carry the page's scope, not the component's. The one exception is the last-row border, which needs tbody :global(tr:last-child td) to reach into the snippet; a scoped selector there matches nothing and is pruned (the compiler reports it as unused CSS).

Checks

svelte-check (0 errors), biome check and build pass, with no unused-CSS warnings. Not exercised against a running server.