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/Select.css

@@ -44,6 +44,25 @@
   -webkit-appearance: none;
 }
 
+/*
+ * The native option popup is OS-rendered: without explicit colours it falls
+ * back to the UA scheme (white) while the option text inherits the themed
+ * (light) colour — i.e. white-on-white in dark mode wherever the surrounding
+ * app keeps color-scheme: light (e.g. the editor, whose dark mode is driven by
+ * Mantine, not color-scheme). Theme the options explicitly and pin the
+ * select's color-scheme to the active SUI theme so the popup is always legible.
+ */
+.sui-select__el option {
+  background-color: var(--color-surface);
+  color: var(--color-text-1);
+}
+[data-theme="dark"] .sui-select__el {
+  color-scheme: dark;
+}
+[data-theme="light"] .sui-select__el {
+  color-scheme: light;
+}
+
 .sui-select--sm .sui-select__el {
   font-size: 0.8125rem;
   padding-left: var(--space-2);