[6.x] Fix/15401 nested interactive controls

[jaygeorge]

Description of the Problem

Three CP spots put a focusable element inside (or next to) a control that already claims an interactive role, which breaks keyboard/AT expectations (#15401):

1. Login — positive tabindex values (1–6) pulled the form ahead of skip links
2. Blueprint tabs — edit/delete controls lived inside role="tab" (nested-interactive)
3. Blueprint tablist — Shopify Draggable’s Focusable plugin added tabindex to non-tab wrappers (aria-required-children)

The Remember me nested-checkbox shape from the issue is already fixed upstream in Reka (hidden input as a sibling).

What this PR Does

• Removes positive login tabindexs so focus follows document order (skip links first)
• Keeps “Forgot password?” visually by the Password label, but last in tab order (after Continue / passkey / OAuth) via CSS anchor positioning — no positive tabindex
• Moves blueprint tab edit/delete UI out of TabTrigger
• Moves overflow menus outside TabList (blueprints + publish tabs)
• Excludes Draggable Focusable on sortable blueprint tabs so wrappers don’t get tabindex inside the tablist

Fixes #15401

How to Reproduce

1. Login tab order — visit /cp/auth/login, Tab from the top of the page
   Before: form controls jump ahead of skip links
   After: skip links (if present), then Email → Password → Remember me → Continue → (passkey/OAuth) → Forgot password?
2. Blueprint nested tab — open /cp/fields/blueprints/collections/{handle}/{blueprint}/edit with 2+ tabs, select a tab so the chevron shows
   document.querySelectorAll('[role="tab"] button').length → 0
3. Blueprint tablist tabindex — same page after load
   document.querySelectorAll('.blueprint-tab[tabindex]').length → 0
4. Overflow outside tablist — narrow until the ⋯ menu appears
   document.querySelector('[role="tablist"]')?.contains(document.querySelector('[aria-label="Open dropdown menu"]')) → false