SearchableTextBlock: animated accordion, tag counts, empty state, focus-visible

- Replace <details> with Svelte-controlled accordion + slide transition (180ms)
- Close all items on search/tag filter change
- Live tag counts per filter showing matches for current search query
- Visual empty state with reset button when no results
- focus-visible ring on all interactive elements (was focus: — shows on click too)
- Tag.svelte: remove ring-0, add focus-visible:ring-2
- Card.svelte: add focus-visible ring to linked cards

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/components/Card.svelte

@@ -47,7 +47,7 @@
     variants: {
       variant: {
         callout: "cursor-pointer transition-colors no-underline text-inherit hover:bg-wald-100",
-        tile: "transition-[transform,box-shadow,border-color] duration-200 no-underline hover:-translate-y-0.5 hover:shadow-md hover:border-wald-300",
+        tile: "transition-[transform,box-shadow,border-color] duration-200 no-underline hover:-translate-y-0.5 hover:shadow-md hover:border-wald-300 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-wald-500 focus-visible:ring-offset-2",
       },
     },
     defaultVariants: { variant: "tile" },