Split Settings into sub-pages and add DNS cache clearing

Settings was a single long page. Break it into a sub-nav (Notifications,
DNS Badges, Cache) with its own route per section, each loading and
saving independently. Also add the DNS record-cache clearing action
that Sloth Manager had — a new admin-only POST /api/dns/cache/clear
truncates the zone/record cache tables so stale data can be wiped and
zones re-synced from scratch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/src/routes/dns.ts

@@ -506,3 +506,19 @@ dnsRouter.delete("/providers/:id/zones/:zoneId/records/:recordId", requireRole("
     res.status(502).json({ error: err instanceof Error ? err.message : String(err) });
   }
 }));
+
+// ─── Cache ───────────────────────────────────────────────────────────────────
+
+dnsRouter.post("/cache/clear", requireRole("admin"), asyncHandler(async (req, res) => {
+  await db.delete(dnsRecordsCache);
+  await db.delete(dnsZonesCache);
+
+  await recordAudit({
+    actor: req.currentUser!,
+    category: "dns",
+    action: "clear_cache",
+    targetType: "dns_cache",
+  });
+
+  res.json({ ok: true });
+}));