chore(ts): replace 7 \as unknown as\ shims with proper types

Most of the existing coercions are legitimate (Drizzle raw-SQL row
shapes, linkedom DOMParser interop, fit-file-parser's loose \`any\`
callback API) — leaving those is honest. The ones cleaned up here
were shims around APIs that have real types we just hadn't wired:

- **\`window.__leafletMap\`** (4 sites: MapHelpers.tsx, SessionView.tsx ×3)
  Added \`Window\` declaration in \`apps/planner/app/types/global.d.ts\`.
  Callers now use plain \`window.__leafletMap\` typed as
  \`L.Map | undefined\`.

- **\`L.markerClusterGroup\`** (PoiPanel.tsx) — leaflet.markercluster
  augments the global \`L\` namespace at runtime but ships no types.
  Added a \`declare module \"leaflet\"\` block in the same global.d.ts
  with a minimal signature for what we actually use.

- **\`L.DomEvent.preventDefault / .stop\`** taking a Leaflet event
  rather than a native one (PlannerMap.tsx, RouteInteraction.tsx,
  NoGoAreaLayer.tsx). Leaflet events carry the native event under
  \`.originalEvent\`; using that drops the cast entirely.

- **\`_creds as unknown as OAuthCredentials\`** orphan in
  wahoo/webhook.ts — \`_creds\` was unused inside the callback (the
  void-cast was a no-op preserving the type for documentation).
  Replaced with \`async ()\`, dropped the unused import.

Net: 26 \`as unknown as\` / \`as any\` sites → 19, all remaining ones
gated by external lib interop or Drizzle raw-SQL.

Full repo: pnpm typecheck / lint / test all green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/planner/app/components/PoiPanel.tsx

@@ -90,8 +90,9 @@ export function PoiMarkers({ poiState, onAddWaypoint }: PoiPanelProps) {
     // Dynamic import to avoid bundling markercluster when POIs aren't used
     import("leaflet.markercluster").then(() => {
       if (!mounted) return;
-      // After import, L.markerClusterGroup is available
-      const cluster = (L as unknown as { markerClusterGroup: (opts?: object) => L.LayerGroup }).markerClusterGroup({
+      // After import, L.markerClusterGroup is available (typed via
+      // module augmentation in app/types/global.d.ts).
+      const cluster = L.markerClusterGroup({
         maxClusterRadius: 40,
         disableClusteringAtZoom: 15,
         showCoverageOnHover: false,