feat: add custom branding for signing pages (#2785)

Platform-plan organisations and teams can now customise non-embed
signing pages with six brand colour tokens, a border-radius, and
a free-text custom CSS block (up to 256 KB).

- Stored on OrganisationGlobalSettings / TeamGlobalSettings;
  teams inherit from the org via brandingEnabled === null.
- CSS is sanitised on save (PostCSS) so we can inline it at SSR
  with no per-render parsing.
- Rendered via a nonce'd <style> scoped under .documenso-branded,
  using native CSS nesting so user selectors don't need scoping.
- Gated on the existing embedSigningWhiteLabel claim (or
  self-hosted) — reuses the embed white-label decision.

package-lock.json

@@ -349,20 +349,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "apps/docs/node_modules/typescript": {
-      "version": "5.9.3",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",
-      "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
-      "dev": true,
-      "license": "Apache-2.0",
-      "bin": {
-        "tsc": "bin/tsc",
-        "tsserver": "bin/tsserver"
-      },
-      "engines": {
-        "node": ">=14.17"
-      }
-    },
     "apps/docs/node_modules/undici-types": {
       "version": "7.19.2",
       "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.19.2.tgz",
@@ -30684,6 +30670,8 @@
         "pino": "^9.14.0",
         "pino-pretty": "^13.1.2",
         "playwright": "1.56.1",
+        "postcss": "^8.5.6",
+        "postcss-selector-parser": "^7.1.0",
         "posthog-js": "^1.297.2",
         "posthog-node": "4.18.0",
         "react": "^18",
@@ -30701,6 +30689,19 @@
         "vitest": "^4.0.18"
       }
     },
+    "packages/lib/node_modules/postcss-selector-parser": {
+      "version": "7.1.1",
+      "resolved": "https://registry.npmjs.org/postcss-selector-parser/-/postcss-selector-parser-7.1.1.tgz",
+      "integrity": "sha512-orRsuYpJVw8LdAwqqLykBj9ecS5/cRHlI5+nvTo8LcCKmzDmqVORXtOIYEEQuL9D4BxtA1lm5isAqzQZCoQ6Eg==",
+      "license": "MIT",
+      "dependencies": {
+        "cssesc": "^3.0.0",
+        "util-deprecate": "^1.0.2"
+      },
+      "engines": {
+        "node": ">=4"
+      }
+    },
     "packages/prettier-config": {
       "name": "@documenso/prettier-config",
       "version": "0.0.0",