fix(nav): keep desktop side nav visible under the user panel backdrop

[dmnyc]

Fixes #426.

Problem

On desktop, opening the right-side user menu made the left side nav and logo vanish completely — the backdrop's dim/blur read as a solid wall over the left column.

Root cause

DesktopSideNav faded itself to opacity-0 + pointer-events-none whenever userSidePanelOpen was set (introduced in f7b488c, the unified-messaging commit). So the nav wasn't merely dimmed by the backdrop — it was removed outright, and the blur showed the feed content behind where it used to be.

Fix

• Remove the self-fade. The nav stays in place at its normal z-index and gets dimmed + blurred by the panel's backdrop uniformly with the rest of the page — the menu and logo remain discernible through the frost, with no hard edges or special-cased regions.
• UserSidePanel now also closes itself on afterNavigate as a safeguard, so the panel can never linger over a new page after navigation.

Test plan

• Desktop (≥1280px): open the avatar menu — left nav + logo visible through the uniform dim/blur (matching the rest of the page) instead of disappearing
• Click the dimmed area — panel closes (click-outside unchanged)
• Mobile width: behavior unchanged (side nav is hidden below xl anyway)
• Regular modals unaffected (nav z-index untouched at z-10)