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/SectionHeader.css

@@ -0,0 +1,33 @@
+.sui-sectionhdr {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+  width: 100%;
+  padding: 0.4rem 0;
+  background: transparent;
+  border: none;
+  text-align: left;
+}
+button.sui-sectionhdr {
+  cursor: pointer;
+}
+.sui-sectionhdr__title {
+  flex: 1;
+  font-size: 0.6875rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  color: var(--color-text-4);
+}
+.sui-sectionhdr__count {
+  font-size: 0.6875rem;
+  color: var(--color-text-4);
+}
+.sui-sectionhdr__chevron {
+  color: var(--color-text-4);
+  flex-shrink: 0;
+  transition: transform var(--motion-fast);
+}
+.sui-sectionhdr__chevron[data-collapsed="true"] {
+  transform: rotate(-90deg);
+}