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/NavItem.tsx

@@ -1,6 +1,8 @@
 import type { ReactNode } from "react";
 import "@shared/components/NavItem.css";
 
+export type NavItemAccent = "blue" | "purple" | "green" | "amber" | "red";
+
 export interface NavItemProps {
   /** Stable view id passed to the click handler. */
   id: string;
@@ -8,6 +10,11 @@ export interface NavItemProps {
   icon?: ReactNode;
   /** Show the active highlight (navActive background, navActiveText colour). */
   isActive?: boolean;
+  /**
+   * Optional status accent: draws a left edge bar in the tone colour and tints
+   * the leading icon to match. For listing live/paused/etc. entities.
+   */
+  accent?: NavItemAccent;
   /** Optional trailing badge (e.g. unread count, "new"). */
   trailing?: ReactNode;
   onClick?: (id: string) => void;
@@ -19,13 +26,14 @@ export interface NavItemProps {
  *
  * Active styling: navActive background, navActiveText colour, weight 500.
  * Hover styling: navHover background, navHoverText colour (only when not
- * already active).
+ * already active). An optional `accent` adds a status edge bar + icon tint.
  */
 export function NavItem({
   id,
   label,
   icon,
   isActive,
+  accent,
   trailing,
   onClick,
   className,
@@ -34,9 +42,15 @@ export function NavItem({
     <button
       type="button"
       onClick={() => onClick?.(id)}
-      className={["sui-navitem", isActive ? "is-active" : "", className ?? ""]
+      className={[
+        "sui-navitem",
+        isActive ? "is-active" : "",
+        accent ? "sui-navitem--accent" : "",
+        className ?? "",
+      ]
         .filter(Boolean)
         .join(" ")}
+      data-accent={accent}
       aria-current={isActive ? "page" : undefined}
     >
       {icon && (