ui: convert ActivityTable to shadcn-svelte data-table

Replace the hand-rolled table with a TanStack Table-backed shadcn
data-table using the FlexRender/createSvelteTable helpers, with
DropdownMenu column visibility, Select page-size, and icon-button
pagination. Column visibility and page size persist to localStorage.

Move tooltip usage to the canonical shadcn-svelte pattern by adding a
single root Tooltip.Provider in App.svelte and using Tooltip.Root/
Trigger/Content directly in the activity-table sub-components
(HeaderLabel, MetaCell), dropping the custom Tooltip/MetadataTooltip
wrappers.

- add @tanstack/table-core and shadcn data-table helpers
- split cell/header renderers into activity-table/* sub-components
- switch pagination/visibility to TanStack Table state driven by
  table.nextPage/previousPage/setPageIndex/setPageSize and
  column.toggleVisibility

ui-svelte/package.json

@@ -34,6 +34,7 @@
     "vitest": "^4.1.0"
   },
   "dependencies": {
+    "@tanstack/table-core": "^8.21.3",
     "chart.js": "4.5.1",
     "highlight.js": "^11.11.1",
     "katex": "^0.16.28",