… lights + restore scoped window-drag (#2127)

* fix(open-knowledge): align fullscreen graph header with macOS traffic lights + restore scoped window-drag

The fullscreen graph overlay (`fixed inset-0`) starts at the raw window top,
8px above the editor chrome row (which sits inside SidebarInset's `m-2`). Its
header now reproduces that 8px inset (`mt-2`), matches the chrome row's `h-12`
band, and drops PanelHeader's inherited `py-3` so content centers on the same
y=32 midline the traffic lights are tuned to (verified by measuring the
rendered rects). A structural spacer reserves the traffic-light footprint so
the title no longer overlaps them horizontally.

The overlay paints over the editor's `-webkit-app-region: drag` chrome. Drag
regions composite at the OS layer and show through an opaque overlay, so clicks
on the Explore/Orphans/Hubs tabs were being converted to window drags. Scope it
like EditorHeader: the header row is the drag region, the controls cluster opts
back out with `[&>*]:no-drag`. The window stays draggable by the header while
the tabs/buttons receive clicks.

* fix(open-knowledge): use precedent-#49 pl- reserve instead of spacer in graph header

Address PR review: the spacer approach removed the `pl-[var(--ok-titlebar-reserve-left,1rem)]`
marker that precedent #49's class-level guard (fullscreen-overlay-safe-area-coverage.test.ts)
requires within ±6 lines of every `fixed inset-0`, which would have failed CI.

Measured the premise the spacer was built on: `pl-[var(--ok-titlebar-reserve-left,1rem)]`
resolves to 78px even with PanelHeader's base `px-4` (the arbitrary `pl-` wins by Tailwind
emit order) — so the cascade-collapse the spacer guarded against does not occur. Reverted to
the precedent-compliant `pl-` reserve on the chrome row; dropped the spacer.

Keeps the vertical fix (`mt-2 h-12 py-0` → title at y=32) and the header-scoped window-drag
(header is the drag region, controls opt out with `[&>*]:[-webkit-app-region:no-drag]`). Also
fixes a comment that quoted the invalid `[&>*]:no-drag` shorthand.

* fix(open-knowledge): restore traffic-light clearance lost in spacer→pl- revert

The spacer stacked on the header's base px-4 (16px + 78px = 94px to the title),
but `pl-` *replaces* px-4's left, giving only 78px — which leaves the title
touching the traffic lights (overlap regression). Re-add the 16px via `ml-4` on
the title cluster, restoring the 94px clearance the spacer version had (measured:
pl-[…]=78px + ml-4=16px → title at 94px, identical to the approved spacer).

Also reword the header comment so it no longer contains the literal `fixed
inset-0` string — the safe-area coverage guard regex scans comment lines, and
the prose was being matched as a spurious class-pattern hit without a nearby
marker once the comment grew.

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GitOrigin-RevId: 6c3f321523cb9f5cef1c047982d3c4232040413c