Round 25: Mega-menu admin nav — every filter visible, nothing hidden

Admin nav restructured to match the Pakk pattern. 9 top tabs (Sales, Inventory, Purchases, Accounting, Web, Files, Reports, Setup, Admin) each open a mega-menu flyout with grouped sub-nav. Every filter, every stage, every preset rendered flat — no popovers, no hidden dropdowns. Kebab `⋮` kept as informational indicator: hover tooltip says the flat list is exhaustive. Top tabs scroll horizontally on narrow viewports instead of truncating. Web-disabled products, missing images, inactive suppliers, expired discounts, failed webhooks, audit errors — all reachable from nav with one click.