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/PanelHeader.tsx

@@ -1,5 +1,6 @@
 import type { ReactNode } from "react";
 import "@shared/components/PanelHeader.css";
+import { IconBadge } from "@shared/components/IconBadge";
 
 export interface PanelHeaderProps {
   title: ReactNode;
@@ -7,6 +8,10 @@ export interface PanelHeaderProps {
   subtitle?: ReactNode;
   /** Show a back chevron and trigger this callback when clicked. */
   onBack?: () => void;
+  /** Optional leading visual (e.g. a category glyph) shown in a tinted box. */
+  icon?: ReactNode;
+  /** Accent tint for the leading icon box. Defaults to blue. */
+  iconAccent?: "blue" | "purple" | "green" | "amber" | "red";
   /** Right-aligned action buttons / chips. */
   actions?: ReactNode;
   className?: string;
@@ -14,12 +19,15 @@ export interface PanelHeaderProps {
 
 /**
  * Header strip used by drill-down panels (admin tabs, agent detail, settings
- * sub-pages). Back chevron renders only when `onBack` is supplied.
+ * sub-pages). Back chevron renders only when `onBack` is supplied; an optional
+ * leading `icon` renders in a tinted box before the title.
  */
 export function PanelHeader({
   title,
   subtitle,
   onBack,
+  icon,
+  iconAccent = "blue",
   actions,
   className,
 }: PanelHeaderProps) {
@@ -49,6 +57,11 @@ export function PanelHeader({
             </svg>
           </button>
         )}
+        {icon && (
+          <IconBadge accent={iconAccent} size="md">
+            {icon}
+          </IconBadge>
+        )}
         <div className="sui-panelhdr__text">
           <div className="sui-panelhdr__title">{title}</div>
           {subtitle && <div className="sui-panelhdr__sub">{subtitle}</div>}