Unify remote element style declarations (#23263)

Front components run in a Web Worker with a fake DOM. Until now the
worker had a hand-rolled `style` object for remote elements and the host
had its own separate CSS-string parser: two implementations of the same
parsing that kept drifting apart (several review rounds fixed edge cases
in one copy but not the other).

What changed:
- One shared `createStyleProxy` now backs `element.style` in the worker,
and one shared `parseCssDeclarations` feeds both the worker proxy and
the host's `parseCssString`. Most of the diff is existing logic split
out of `installStylePropertyOnRemoteElements` into small single-purpose
utils (`splitCssDeclarations`, `stripImportantPriorityFromCssValue`,
`normalizeCssPropertyName`, `formatCssValue`, ...), not new behavior.
- `!important` is stripped from values instead of tracked. Nothing ever
read priorities back, and the host applies styles through React inline
styles, which cannot express `!important`. Rendering note: `color: red
!important` used to reach React as an invalid value (property silently
not applied); it now applies, without the priority.
- Style writes flush to the host synchronously, exactly as on main.
- The parser handles quotes, escapes and parentheses; CSS comments
inside hand-written `cssText` are not supported.

This shared proxy is also the base for the worker `getComputedStyle`
stub in the geometry PR. Second of three PRs splitting the geometry
mirror work.

packages/twenty-front-component-renderer/src/utils/__tests__/isCssCustomPropertyName.test.ts

@@ -0,0 +1,11 @@
+import { isCssCustomPropertyName } from '../isCssCustomPropertyName';
+
+describe('isCssCustomPropertyName', () => {
+  it('should detect a custom property name', () => {
+    expect(isCssCustomPropertyName('--gap')).toBe(true);
+  });
+
+  it('should reject a standard property name', () => {
+    expect(isCssCustomPropertyName('color')).toBe(false);
+  });
+});