fix(twenty-front): new layout fast-follows (#21360)

Fast-follows for the new layout / flat redesign (master:
twentyhq/core-team-issues#2478).

## Changes
- **Main navbar 48px** (twentyhq/core-team-issues#2479) —
`SIDE_PANEL_TOP_BAR_HEIGHT` 40 → 48, so `PageCardHeader` matches the
Figma target. The side panel top bar shares this constant and stays
aligned.
- **Content panel 12px radius** (twentyhq/core-team-issues#2480) —
`PageCardLayout` card gets a full border + 12px radius and an 8px inset
(`spacing[2]`) so it floats on the shell instead of square full-bleed.
- **Square three-dots button** (twentyhq/core-team-issues#2481) — added
a `square` option to `AnimatedButton`; the page-header side-panel toggle
now renders a 24×24 square icon button instead of a 32×24 pill.
- **Table checkbox sizing** (twentyhq/core-team-issues#2482) — restored
`box-sizing: content-box` on the checkbox box. Its border is declared
outside the label size, so the global `border-box` reset (#21349) was
shrinking it (14px → 12px). Same fix pattern as #21349.
- **Tertiary navbar background** (twentyhq/core-team-issues#2483) — left
navbar / app shell use `background/tertiary` instead of the noisy
surface (`DefaultLayout`, `UserOrMetadataLoader`).
- **Skeleton loading** (twentyhq/core-team-issues#2484) — metadata +
content loading now match the new layout: tertiary shell, 12px rounded
content panel, 48px navbar, sparse bars, empty body (removed the dense
full-width rows). Left-panel skeleton bars use `quaternary` so they stay
visible on the tertiary shell.

## Verification
- typecheck (tsgo) + oxlint + oxfmt pass for all changed files.
- Verified live against a running workspace: measured navbar = 48px,
three-dots = 24×24, checkbox box-sizing = content-box (14px), card
radius = 12px, shell background = tertiary (no noisy image).
Content-loading skeleton matches the target.

packages/twenty-front/src/modules/ui/layout/page/components/DefaultLayout.tsx

@@ -25,7 +25,7 @@ import { AnimatePresence, LayoutGroup } from 'framer-motion';
 import { Outlet } from 'react-router-dom';
 import { themeCssVariables } from 'twenty-ui-deprecated/theme-constants';
 const StyledLayout = styled.div`
-  background: ${themeCssVariables.background.noisy};
+  background: ${themeCssVariables.background.tertiary};
   display: flex;
   flex-direction: column;
   height: 100dvh;

packages/twenty-front/src/modules/ui/layout/page/components/PageCardLayout.tsx

@@ -28,6 +28,7 @@ const StyledMainCardWrapper = styled.div`
 const StyledCard = styled.div`
   background: ${themeCssVariables.background.primary};
   border-left: 1px solid ${themeCssVariables.border.color.medium};
+  border-radius: 12px 0 0 12px;
   border-right: 1px solid ${themeCssVariables.border.color.medium};
   box-sizing: border-box;
   display: flex;