fix: preserve value types in extraFields (no string coercion)

- DocumentMeta.extraFields type -> Record<string, unknown>
- API routes pass extraFields through without JSON.stringify coercion
- CustomFieldBadges auto-styles by value type/content (number->purple, boolean->green/red, etc.)
- DocForm help text: generic (no hardcoded field examples)

content/notes/test/dynamic-field-types.md

@@ -0,0 +1,19 @@
+---
+title: "Dynamic Field Types Test"
+type: "note"
+section: "notes"
+status: "published"
+author: "asep"
+tags: ["test"]
+path: "notes/test/dynamic-field-types.md"
+created_at: "2026-08-20T17:45:07.294Z"
+updated_at: "2026-08-20T17:45:07.294Z"
+priority: "5"
+resolved: "true"
+os: "linux"
+complexity: "medium"
+tags_list: "[\"alpha\",\"beta\"]"
+metadata: "{\"owner\":\"team-a\",\"tier\":2}"
+---
+# Dynamic Field Types Test
+Testing various field value types.