feat(web): alerts sections move to a left rail - #2361

🟢 Confidence 10/10 · safe to merge. Contained UI change: the rail, its aria-current, href search-param handling and breadcrumbs are pinned by a new scene test, and every navShell importer was moved.
_{no findings · tests covered · risk medium}

Moves the alerts hub's eight sections from a tab strip into the shared left rail (navShell, relocated to page/kit with an optional trailing count), names the page and breadcrumbs after the open section, and folds the rail into the top bar's "Alerts" sheet below lg. Faithful to the old tabs.

• navShell moves to page/kit and rows gain an optional count with muted/error tone
• /alerts renders its sections in the filter rail, titled Alerts › <section>
• Rail rows rebuild the same search params the old tabs produced
• Alerts labs take their breadcrumbs from hubBreadcrumbs

_{0c556b1 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}