fix(web): window/table polish — opaque windows, sortable columns, sticky-header scrollbar, viewport-clamped windows

- Windows: make floating windows fully opaque (drop backdrop-blur/color-mix
  transparency), add margin around windows, reduce Overview padding to p-2.
- DataTable: fix Toolbar always rendering an empty padded bar (children
  slot was always truthy regardless of actual content); split header/body
  into separate tables so the scrollbar no longer overlaps the sticky
  header; make sort work for derived/synthetic columns by sorting on the
  column's accessor instead of a nonexistent row key.
- Overview: enable sorting on Status and Task columns via accessors.
- TaskContextPanel: give the Activity pane more height by default (Scope
  30% / Activity 70%), fixing that the saved split sizes were never
  actually applied to the bound Pane sizes.
- windows.ts: clamp new/resized windows to the desktop viewport so
  content-heavy entity windows can't grow taller than the visible screen;
  fixes a bad defaultSize.height ('30vh', an invalid non-numeric value)
  that had silently left window height unconstrained.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/pages/Overview.svelte

@@ -53,7 +53,7 @@
   })
 
   const columns: DataTableColumn<Session>[] = [
-    { key: '_status', header: 'Status', render: StatusDotRenderer, width: '140px' },
+    { key: '_status', header: 'Status', render: StatusDotRenderer, sortable: true, accessor: (s) => bucket(s), width: '140px' },
     { key: '_heading', header: 'Task', sortable: true, accessor: heading, truncate: true },
     { key: 'summary', header: 'Summary', accessor: (s) => s.summary || '—', truncate: true, headerClass: 'hidden md:table-cell', class: 'hidden md:table-cell' },
     { key: 'last_active_at', header: 'Last active', render: 'relative-time', sortable: true, width: '112px', align: 'right' },
@@ -66,7 +66,7 @@
   )
 </script>
 
-<div class="relative flex h-full flex-col gap-3 overflow-hidden p-4">
+<div class="relative flex h-full flex-col gap-3 overflow-hidden p-2">
 
   <div class="relative z-10 flex flex-wrap items-center gap-1.5">
     {#each FILTERS as f}