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.

packages/ui/primitives/color-picker.tsx

@@ -1,7 +1,7 @@
 import type { HTMLAttributes } from 'react';
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 
-import { HexColorInput, HexColorPicker } from 'react-colorful';
+import { HexColorInput, HexColorPicker, setNonce } from 'react-colorful';
 
 import { cn } from '../lib/utils';
 import { Popover, PopoverContent, PopoverTrigger } from './popover';
@@ -11,6 +11,7 @@ export type ColorPickerProps = {
   value: string;
   defaultValue?: string;
   onChange: (color: string) => void;
+  nonce?: string;
 } & HTMLAttributes<HTMLDivElement>;
 
 export const ColorPicker = ({
@@ -19,6 +20,7 @@ export const ColorPicker = ({
   value,
   defaultValue = '#000000',
   onChange,
+  nonce,
   ...props
 }: ColorPickerProps) => {
   const [color, setColor] = useState(value || defaultValue);
@@ -39,6 +41,12 @@ export const ColorPicker = ({
     onChange(inputColor);
   };
 
+  useEffect(() => {
+    if (nonce) {
+      setNonce(nonce);
+    }
+  }, [nonce]);
+
   return (
     <Popover>
       <PopoverTrigger>
@@ -57,6 +65,7 @@ export const ColorPicker = ({
           color={color}
           onChange={onColorChange}
           aria-disabled={disabled}
+          nonce={nonce}
           {...props}
         />
 
@@ -72,6 +81,7 @@ export const ColorPicker = ({
             }
           }}
           disabled={disabled}
+          nonce={nonce}
         />
       </PopoverContent>
     </Popover>