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.css

@@ -16,16 +16,22 @@
   display: inline-flex;
   align-items: center;
   justify-content: center;
-  width: 2rem;
-  height: 2rem;
-  border-radius: var(--radius-md);
-  color: var(--color-text-3);
+  flex-shrink: 0;
+  width: 1.875rem;
+  height: 1.875rem;
+  border: 1px solid var(--color-border);
+  border-radius: 50%;
+  background: var(--color-surface);
+  color: var(--color-text-2);
+  cursor: pointer;
   transition:
     background var(--motion-fast),
-    color var(--motion-fast);
+    color var(--motion-fast),
+    border-color var(--motion-fast);
 }
 .sui-panelhdr__back:hover {
   background: var(--color-bg-hover);
+  border-color: var(--color-text-4);
   color: var(--color-text-1);
 }
 .sui-panelhdr__text {