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/Checkbox.tsx

@@ -7,13 +7,15 @@ export interface CheckboxProps extends Omit<
 > {
   label?: ReactNode;
   description?: ReactNode;
+  /** Optional glyph shown between the box and the label text. */
+  leadingIcon?: ReactNode;
   /** Tri-state checkbox visual (still focusable + submittable, but renders the dash). */
   indeterminate?: boolean;
 }
 
 export const Checkbox = forwardRef<HTMLInputElement, CheckboxProps>(
   function Checkbox(
-    { label, description, indeterminate, className, id, ...rest },
+    { label, description, leadingIcon, indeterminate, className, id, ...rest },
     ref,
   ) {
     return (
@@ -57,6 +59,11 @@ export const Checkbox = forwardRef<HTMLInputElement, CheckboxProps>(
           </svg>
           <span className="sui-check__dash" />
         </span>
+        {leadingIcon && (
+          <span className="sui-check__icon" aria-hidden>
+            {leadingIcon}
+          </span>
+        )}
         {(label || description) && (
           <span className="sui-check__text">
             {label && <span className="sui-check__label">{label}</span>}