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.stories.tsx

@@ -0,0 +1,36 @@
+import { useState } from "react";
+import type { Meta, StoryObj } from "@storybook/react-vite";
+import { SectionHeader } from "@shared/components/SectionHeader";
+
+const meta: Meta<typeof SectionHeader> = {
+  title: "Primitives/SectionHeader",
+  component: SectionHeader,
+  tags: ["autodocs"],
+  parameters: { layout: "padded" },
+  decorators: [
+    (S) => (
+      <div style={{ maxWidth: "20rem" }}>
+        <S />
+      </div>
+    ),
+  ],
+};
+export default meta;
+type Story = StoryObj<typeof SectionHeader>;
+
+export const Static: Story = { args: { title: "Policies", count: "3 active" } };
+
+export const Collapsible: Story = {
+  render: () => {
+    const [open, setOpen] = useState(true);
+    return (
+      <SectionHeader
+        title="Policies"
+        count="3 active"
+        collapsible
+        expanded={open}
+        onToggle={() => setOpen((v) => !v)}
+      />
+    );
+  },
+};