feat(alerts): alerting sections as real pages in the sidebar - #2426

🟢 Confidence 10/10 · safe to merge. Routing, redirects and the silence panel check out; the hub and rule scene tests plus editor/rule/lab files went unread here.
_{no findings · tests covered · risk medium}

Each alerting section now lives at its own /alerts/<section> path with sidebar sub-items, and the new-silence form became a URL-held side panel. The redirect and permission paths I could check hold up; nothing to fix before merge.

• urlToAppRoute parses AlertsSection and keeps /alerts/rules/<id> reachable; AlertRuleLegacy redirects old rule links
• routeRedirect turns a legacy ?tab= into its section path, keeping the other params
• followSilenceSheet syncs the dialog to ?sheet=silence, dropping the param for viewers without alerts:manage
• matchSubItem/activeSubHrefOf pick the most specific sub-item, including alsoActive pages

_{6a9012e · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}