feat(blocks): live_strommix widget for energy-charts data

Editor-controlled live-strommix card that surfaces what share of
electricity demand is currently covered by weather-dependent
renewables (Wind+Solar / Load), plus the conventional generation that
fills the gap, the cross-border saldo, and the day-ahead price (DE/FR
side-by-side). Status bucket + label + argument come from the CMS so
editors can iterate copy without a deploy.

- LiveStrommixBlockData type next to the other block-types.
- New `/api/strommix` aggregator route bundles four CMS external
  collections (strommix_de, crossborder_de, price_de, price_fr) into a
  single payload, computes the renewable + conventional sums and the
  cross-border saldo (Σ flow_xx_gw, sign convention: + = export from
  DE), surfaces stale-on-error flags from the CMS response headers.
  Cache-control 5min so a single node only hits energy-charts once
  per window.
- StrommixBlock.svelte renders the card: percentage, bucket label,
  bucket argument (markdown), context row (conventional + saldo with
  ↑/↓ arrow), price row with optional FR comparison + negative-price
  badge, footer with disclaimer + "Wie wird das berechnet?" link +
  stale-data hint when the CMS served fallback. Re-polls every 5min
  client-side; bucket thresholds + labels + arguments come from the
  block instance so multiple placements can carry different copy.
- ContentRows.svelte dispatcher branch on `_type === "live_strommix"`.

Wired against `cms_content/_types/core/live_strommix.json5` (added in
the rustycms repo today). content_layout.row{1,2,3}Content already
allow the new block-type via the same commit on the CMS side.

src/lib/strommix.ts

@@ -0,0 +1,35 @@
+/**
+ * Shared types for the Strommix-Live-Widget.
+ *
+ * The aggregator route at `/api/strommix` builds this payload from the
+ * four CMS external collections; the `StrommixBlock.svelte` component
+ * consumes it.
+ */
+
+export type StrommixResponse = {
+  measuredAtUnix: number | null;
+  // Production (MW)
+  windOnshoreMw: number;
+  windOffshoreMw: number;
+  solarMw: number;
+  biomassMw: number;
+  hydroMw: number;
+  ligniteMw: number;
+  hardCoalMw: number;
+  gasMw: number;
+  nuclearMw: number;
+  // Aggregates (MW)
+  loadMw: number;
+  weatherRenewableMw: number;
+  conventionalMw: number;
+  // Cross-border (GW; positive = net export from DE, negative = import)
+  netFlowGw: number;
+  flowsByCountry: Record<string, number>;
+  // Prices (EUR/MWh; null if upstream unavailable)
+  priceDeEurMwh: number | null;
+  priceFrEurMwh: number | null;
+  // Stale flags surfaced from the CMS' `x-rustycms-external-stale` header
+  staleStrommix: boolean;
+  staleCrossborder: boolean;
+  stalePrice: boolean;
+};