fix(header): mobile Navigate menu opens the nav popover (#1652)

[jung-thomas]

Fixes #1652.

Problem

On mobile / narrow viewports the ui5-shellbar collapses items into its built-in overflow (burger) popover. Tapping Navigate inside that overflow menu closed the menu but never opened the navigation popover — the reporter saw "menu just closes, nothing happens". Desktop (item shown as a button in the bar) worked fine.

Root cause

In overflow, the clicked item's click event reports targetRef = the transient ui5-li inside the shellbar's overflow popover (ShellBarItemTemplate.js renders the item as a ListItemStandard when inOverflow). That overflow popover closes itself on the same click (ShellBar.handleOverflowItemClick). Our handler then set navPopover.opener = <that list item> and opened — but the opener is being detached as the overflow popover closes, and a ui5-popover cannot anchor to a detached/hidden opener, so it silently no-ops. The same latent bug affected Share in overflow.

Fix

hugo/layouts/partials/header.html: detect the overflow case (ShellBarItem.inOverflow, or a ui5-li targetRef) and anchor the nav/share popover to the always-present overflow button ([data-ui5-stable="overflow"]) instead, deferring the open one animation frame so it lands after the overflow popover has closed (avoids a two-popover focus race). Desktop behavior is unchanged — it still anchors to the specific item button.

Tests

• test/e2e/header-mobile-nav.test.js — drives the real click path at a 390px phone viewport and asserts #sb-nav-popover actually opens. Self-skips without SMOKE_BASE_URL; runs in the post-DEV-deploy e2e CI job.
• test/unit/hugo/header-mobile-nav-overflow.test.ts — pins the fix so a future edit can't silently revert it (runs in npm test).

Verification

• Root cause traced conclusively through @ui5/webcomponents-fiori 2.23.2 source.
• Unit guard: confirmed red on origin/main (0 fix tokens) → green with the fix; full test/unit/hugo/ suite 52/52 pass.
• Behavioral e2e requires a deployed env to execute (self-skips locally) — will run against DEV post-deploy. Happy to deploy to DEV for a live mobile check on request.