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.css

@@ -40,8 +40,53 @@
 .sui-navitem__trailing {
   display: inline-flex;
   align-items: center;
+  margin-left: auto;
 }
 .sui-navitem:focus-visible {
   outline: 0.125rem solid var(--color-blue);
   outline-offset: 0.125rem;
 }
+
+/* ---- Status accent (optional) ---- */
+.sui-navitem--accent {
+  position: relative;
+}
+.sui-navitem--accent::before {
+  content: "";
+  position: absolute;
+  left: 0;
+  top: 0.3125rem;
+  bottom: 0.3125rem;
+  width: 3px;
+  border-radius: 3px;
+}
+.sui-navitem[data-accent="blue"]::before {
+  background: var(--color-blue);
+}
+.sui-navitem[data-accent="purple"]::before {
+  background: var(--color-purple);
+}
+.sui-navitem[data-accent="green"]::before {
+  background: var(--color-green);
+}
+.sui-navitem[data-accent="amber"]::before {
+  background: var(--color-amber);
+}
+.sui-navitem[data-accent="red"]::before {
+  background: var(--color-red);
+}
+.sui-navitem[data-accent="blue"] .sui-navitem__icon {
+  color: var(--color-blue);
+}
+.sui-navitem[data-accent="purple"] .sui-navitem__icon {
+  color: var(--color-purple);
+}
+.sui-navitem[data-accent="green"] .sui-navitem__icon {
+  color: var(--color-green);
+}
+.sui-navitem[data-accent="amber"] .sui-navitem__icon {
+  color: var(--color-amber);
+}
+.sui-navitem[data-accent="red"] .sui-navitem__icon {
+  color: var(--color-red);
+}