fix(ui): keep row action menu anchored and stop scroll snap on /app/manage

[localai-bot]

Problem

On the Manage dashboard (/app/manage), clicking the three-dots (kebab) action menu on a model row snapped the page scroll to the top of the window and rendered the menu detached from its trigger, making it impossible to operate.

Root cause

The kebab uses the shared ActionMenu → Popover component. Two compounding defects:

1. Scroll snap to top. The menu auto-focus called el.focus() without preventScroll, so the browser scrolled the focused element into view, yanking the page to the top.
2. Menu mispositioned / unusable. The position: fixed popover was rendered inline inside the table row. The editorial UI overhaul added hover transforms to rows/cards, and a transformed ancestor re-anchors position: fixed to itself instead of the viewport, so the menu (positioned from the trigger's viewport rect) landed in the wrong spot.

Fix

• Portal the Popover to document.body (createPortal) so position: fixed always resolves against the viewport, regardless of ancestor transforms.
• useLayoutEffect for positioning so it's placed before paint (no {0,0} flash).
• preventScroll: true on both focus calls (menu auto-focus + return-focus to trigger).

Testing

New e2e regression test manage-action-menu-position.spec.js:

• Red on old code: scroll jumped from 564 → 0 when opening the menu.
• Green with the fix: scroll stays put, the menu tracks its trigger, and the popover is portaled to body.

All existing ActionMenu/Popover specs still pass (manage-logs-link, nodes-per-node-backend-actions, backends-management, middleware-page, model-editor-back-nav, page-render-smoke — 36 tests).

Assisted-by: Claude:claude-opus-4-8 [Claude Code]