swarm-ui: table filters move to a per-header icon + popover

Replaces the permanent filter-row under Table's headers with a small
filter-icon button in each filterable column's own header cell. The
icon fades in on header hover/focus via a CSS opacity transition, or
stays visible outright once that column actually has a filter set
(mara: "instead of a filter row, add little filter icons on header
hover with fade in out animation ... when a filter is set, the filter
icon does not disappear"). Clicking it opens a small anchored popover
directly under the header holding the exact same filter control
filterMode already provides (text input or select) -- the underlying
filter mechanism from hyperhive#4088 is unchanged, only where the
control lives moved. Close-on-outside-click/Escape mirrors the
contract Dropdown already gives its own popover, adapted to a shared
listener across every column instead of a ref per column since only
one popover is ever open at a time.

New FilterIcon in @hive/shared's icons.tsx (a plain inline SVG funnel,
same Feather/lucide-style shape as the existing GearIcon/LinkIcon) --
found and reused that pattern rather than reaching for an emoji glyph,
matching the documented reason those two exist as SVG in the first
place (mara, on the old emoji icons: inconsistent size/weight across
platforms).

frontend/packages/shared/src/icons.tsx

@@ -56,3 +56,26 @@ export function LinkIcon() {
     </svg>
   );
 }
+
+// A funnel, the standard "filter" glyph — smaller than the other two
+// (1em not 1.3em) since its first caller sits inside a table header
+// cell, not a nav trigger button; scale via the caller's own
+// `font-size` like the others rather than a hardcoded size prop, same
+// reasoning as `GearIcon`/`LinkIcon` above (`currentColor` stroke).
+export function FilterIcon() {
+  return (
+    <svg
+      width="1em"
+      height="1em"
+      viewBox="0 0 24 24"
+      fill="none"
+      stroke="currentColor"
+      stroke-width="2"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+      aria-hidden="true"
+    >
+      <polygon points="22 3 2 3 10 12.46 10 19 14 21 14 12.46 22 3" />
+    </svg>
+  );
+}