feat(twenty-front): generalize the page primary/secondary bars (flat redesign) (#21308)

Replaces #21279 and #21282 with one clean PR from `main`.

Generalizes the settings primary-bar / secondary-bar card chrome to the
record index, record show and standalone pages via a shared
`PageCardLayout` + `PageCardHeader` (the side panel sits as a sibling of
the content card), and applies the new flat design direction: square
corners on the card, side panel and loading skeletons.

Iterating toward the new design (Figma node 102282-221623); the
confirmed direction and the explicit "remove rounded corners" change are
in, remaining designer specifics to follow.

packages/twenty-front/src/modules/side-panel/components/SidePanelForDesktop.tsx

@@ -37,8 +37,6 @@ const StyledSidePanelWrapper = styled.div<{
 
 const StyledSidePanel = styled.aside`
   background: ${themeCssVariables.background.primary};
-  border: 1px solid ${themeCssVariables.border.color.medium};
-  border-radius: ${themeCssVariables.border.radius.md};
   box-sizing: border-box;
   display: flex;
   flex-direction: column;
@@ -58,8 +56,6 @@ const StyledModalContainer = styled.div`
   z-index: 1;
 `;
 
-const GAP_WIDTH = 8;
-
 export const SidePanelForDesktop = () => {
   const isSidePanelOpened = useAtomStateValue(isSidePanelOpenedState);
   const isSidePanelClosing = useAtomStateValue(isSidePanelClosingState);
@@ -130,7 +126,7 @@ export const SidePanelForDesktop = () => {
         currentWidth={sidePanelWidth}
         onWidthChange={handleWidthChange}
         onCollapse={handleCollapse}
-        gapWidth={isSidePanelOpened ? GAP_WIDTH : 0}
+        gapWidth={0}
         cssVariableName={SIDE_PANEL_WIDTH_VAR}
         onResizeStart={handleResizeStart}
       />

packages/twenty-front/src/modules/side-panel/components/SidePanelToggleButton.tsx

@@ -15,10 +15,7 @@ import {
   TooltipPosition,
 } from 'twenty-ui-deprecated/display';
 import { AnimatedButton } from 'twenty-ui-deprecated/input';
-import {
-  getOsControlSymbol,
-  useIsMobile,
-} from 'twenty-ui-deprecated/utilities';
+import { useIsMobile } from 'twenty-ui-deprecated/utilities';
 import {
   ThemeContext,
   themeCssVariables,
@@ -157,7 +154,6 @@ export const SidePanelToggleButton = () => {
           size={isMobile ? 'medium' : 'small'}
           variant="secondary"
           accent="default"
-          hotkeys={[getOsControlSymbol(), 'K']}
           ariaLabel={ariaLabel}
           onClick={toggleSidePanelMenu}
           animate={{