ui: unify SidebarLayout drawer overlay with Drawer (focus trap)

[mgoldsborough]

Deferred from #12 (flagged in QA).

SidebarLayout's drawer collapse mode hand-rolls its own off-canvas overlay (transform + scrim, in SidebarLayout.tsx): it has Escape and inert when closed, but no focus trap and no focus-into on open. The Drawer component now contains focus itself — a hand-rolled Tab trap it grew once the sandbox-safe rebuild (#39) replaced native <dialog>, which had given focus containment for free. So the two off-canvas overlays have diverged on accessibility, and the SidebarLayout one is the less-accessible path.

Goal: extract the Drawer's overlay behavior (focus-into on open + Tab containment + scroll-lock + Escape) into a shared primitive, and have SidebarLayout's drawer mode reuse it — one accessible overlay, no drift. (The remedy is extraction of the now hand-rolled trap, not "reuse the <dialog>-backed Drawer" — <dialog> is unavailable in the app iframe sandbox.)

Not dangerous today (the two don't share code that could silently diverge), but the no-focus-trap path shouldn't linger.

[mgoldsborough]

Correct the direction — this currently prescribes the wider version of the bug

Surfaced by review on #41 (round 9), and it depends on #43's re-scope.

This issue says:

"extract the Drawer's overlay behavior (focus-into on open + Tab containment + scroll-lock + Escape) into a shared primitive, and have SidebarLayout's drawer mode reuse it"
"The remedy is extraction of the now hand-rolled trap, not 'reuse the <dialog>-backed Drawer'"

That routes the next person into propagating a trap that has taken #41 nine rounds and still has a live hole. The hand-rolled tabbability computation has to enumerate browser focus rules that the only available test environment cannot express — six consecutive commits each added one exclusion category, and a 7th (nested closed <details>) is open right now.

SidebarLayout is the easy case, not the one needing the trap

It already carries inert (SidebarLayout.tsx:213) with a comment naming precisely this concern, and its background is a single sibling (SidebarLayout.Main). So containment there is a one-attribute change — inert on the main region while the drawer is open — with no extraction and no trap.

The divergence this issue flags is real: SidebarLayout's drawer has no focus containment and no focus-into on open. Both should be fixed. The correction is only to the remedy: don't extract the hand-rolled trap. Per #43, Drawer should move toward inert — the thing SidebarLayout already does — not the reverse.

Revised goal

1. inert the background in SidebarLayout's drawer mode (one attribute; its background is already a single sibling).
2. Add focus-into-on-open, which is genuinely missing and genuinely shared.
3. Extract a shared primitive only after Drawer: full focus containment (body-recovery + portal-aware owner stack) #43 lands the inert approach, so what gets shared is the durable mechanism rather than the enumeration.

Blocked on #43.