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/MetricCard.tsx

@@ -14,6 +14,8 @@ export interface MetricCardProps {
   deltaDirection?: DeltaDirection;
   /** Visual emphasis. `primary` = darker surface, used for hero metrics. */
   emphasis?: "default" | "primary";
+  /** Density. `sm` tightens padding + value size for narrow rails/strips. */
+  size?: "sm" | "md";
   /** Optional icon shown in the top-right corner. */
   icon?: ReactNode;
   onClick?: () => void;
@@ -41,6 +43,7 @@ export function MetricCard({
   delta,
   deltaDirection,
   emphasis = "default",
+  size = "md",
   icon,
   onClick,
   className,
@@ -50,6 +53,7 @@ export function MetricCard({
 
   const classes = [
     "sui-metric",
+    `sui-metric--${size}`,
     emphasis === "primary" ? "sui-metric--primary" : "",
     interactive ? "sui-metric--interactive" : "",
     className ?? "",