feat(policies): backend-driven policy enforcement (frontend) (#6598)

## Summary
Adds the **Policies** feature (proprietary, behind the
`POLICIES_ENABLED` flag): backend-driven enforcement that runs a fixed
tool pipeline on documents, docked in the right tool sidebar alongside
Tools.

## Highlights
- **Policy catalog** — 5 categories; **Security** is wired (redact PII +
sanitize), the others are marked "Coming soon".
- **Backend as source of truth** — policies persist via the Policies
engine (`/api/v1/policies`), one policy per category, with a local cache
+ offline fallback.
- **Auto-run** — enabled policies run on every uploaded file: dispatch →
poll → import outputs into the workspace.
- **Security redact config** — PII preset dropdown + custom word/regex
entry + advanced options; tool params map to the backend endpoint
fields.
- **Activity feed** with retry on failures; **file badges** showing
which policies ran on a file (sidebar + files page), tinted to the
policy colour.
- Reuses the **Watched Folders** engine for each policy's backing
folder; policy-owned folders are filtered out of the Watched Folders UI.

## Notes
- Gated by `POLICIES_ENABLED` (true in proprietary, false in core) —
unreachable in the open-source build.
- Frontend-only diff; depends on the backend Policies engine and the
merged Watched Folders feature.

frontend/shared/components/SectionHeader.tsx

@@ -0,0 +1,66 @@
+import type { ReactNode } from "react";
+import "@shared/components/SectionHeader.css";
+
+export interface SectionHeaderProps {
+  /** Uppercase eyebrow title. */
+  title: ReactNode;
+  /** Optional right-aligned tally/count. */
+  count?: ReactNode;
+  /** Render as a button with a disclosure chevron. */
+  collapsible?: boolean;
+  /** When collapsible, whether the section is expanded. */
+  expanded?: boolean;
+  onToggle?: () => void;
+  className?: string;
+}
+
+/**
+ * An uppercase section eyebrow with an optional trailing count and, when
+ * `collapsible`, a disclosure chevron + toggle. Use above a group of rows/cards.
+ */
+export function SectionHeader({
+  title,
+  count,
+  collapsible,
+  expanded = true,
+  onToggle,
+  className,
+}: SectionHeaderProps) {
+  const classes = ["sui-sectionhdr", className ?? ""].filter(Boolean).join(" ");
+  const inner = (
+    <>
+      <span className="sui-sectionhdr__title">{title}</span>
+      {count != null && <span className="sui-sectionhdr__count">{count}</span>}
+      {collapsible && (
+        <svg
+          className="sui-sectionhdr__chevron"
+          data-collapsed={!expanded}
+          viewBox="0 0 24 24"
+          width={14}
+          height={14}
+          fill="none"
+          stroke="currentColor"
+          strokeWidth={2}
+          strokeLinecap="round"
+          strokeLinejoin="round"
+          aria-hidden
+        >
+          <polyline points="6 9 12 15 18 9" />
+        </svg>
+      )}
+    </>
+  );
+
+  return collapsible ? (
+    <button
+      type="button"
+      className={classes}
+      onClick={onToggle}
+      aria-expanded={expanded}
+    >
+      {inner}
+    </button>
+  ) : (
+    <div className={classes}>{inner}</div>
+  );
+}