fix(base): restore position: relative on .headerCell

The sticky-headers refactor (cd8d1e0e) moved the header row's sticky
behavior up to .stickyBand and dropped the position rule from
.headerCell entirely — but the cell still has two absolutely-positioned
children that need it as their containing block:

  - .resizeHandle (right: 0) — without a per-cell containing block, all
    handles resolve up to .stickyBand and stack at the band's right
    edge, so only one is visible.
  - Popover.Target (inset: 0) — every header's popover anchor collapses
    to the same band-relative box, so the property menu opens at one
    fixed spot regardless of which header was clicked.

.headerCellPinned still establishes a containing block via position:
sticky, which is why the primary Title column kept working. Re-adding
position: relative on the base .headerCell fixes both the resize handle
and the property-menu popover for non-pinned columns; pinned cells are
unaffected because position: sticky later in the cascade still wins.

apps/client/src/features/base/styles/grid.module.css

@@ -118,6 +118,16 @@
 }
 
 .headerCell {
+  /* Containing block for absolutely-positioned children: the
+   * .resizeHandle (right: 0) at the cell edge, and the Popover.Target
+   * (inset: 0) used to anchor the property menu. Without this, both
+   * resolve up to .stickyBand and (a) all the resize handles stack at
+   * the band's right edge with only one visible, (b) every property
+   * menu opens at the same band-relative spot regardless of which
+   * column was clicked. .headerCellPinned overrides with position:
+   * sticky which also establishes a containing block, so pinned cells
+   * (e.g. the primary Title) kept working without this rule. */
+  position: relative;
   display: flex;
   align-items: center;
   gap: 6px;