fix(strommix): SMARD as primary source — accuracy parity with audit

Audit at 22:15 MESZ flagged the conventional sum (lignite + hard coal
+ gas + nuclear) as 19.1 GW in the widget vs 15.65 GW in reality. The
breakdown showed the gap was Energy-Charts' `Fossil gas` (6.9 GW)
which bundles natural gas + GuD + similar, while SMARD's `Erdgas`
filter 4069 (3.8 GW) tracks just natural gas the way the widget label
implies. SMARD is the canonical German grid feed; switching the
primary source resolves the discrepancy directly.

- New `lib/smard.ts` fetches the 11 relevant filter IDs in parallel:
  conventional (1223/4070/4069/1224), wind (4067/1225), solar (1226),
  hydro (1228), biomass (4066), other-conventional + pumped storage
  (1227/4071, surfaced separately), load (410). Two-step protocol
  (index.json → archive.json) handled inside; output is MW (MWh × 4)
  aligned to one common slot timestamp so every value on screen comes
  from the same point in time.
- Aggregator route uses SMARD for production + load, Energy-Charts
  only for cross-border saldo + day-ahead prices (where SMARD has no
  convenient single-call endpoint). `dataSource: "smard" |
  "energy-charts"` flag travels in the response so the widget can
  attribute the numbers and fall back gracefully when SMARD is down.
- StrommixBlock gets a "Werte prüfen" disclosure that lists every
  component (wind onshore + offshore, solar, the four conventional
  buckets, biomass, hydro, load) with its MW value and the slot
  timestamp + data source. Fact-check users can verify the addition
  themselves now without asking.

Verified live: SMARD @ 21:00 UTC = lignite 8484 + hard coal 3480 +
gas 3811 + nuclear 0 = 15.78 GW conventional. Matches the user's
ground-truth 15.65 GW within sampling noise.

src/lib/strommix.ts

@@ -11,6 +11,10 @@ export type StrommixResponse = {
   // the rest of the values. Cross-border + prices may have their own,
   // earlier or later, timestamps.
   measuredAtUnix: number | null;
+  /** Where production + load came from. `smard` is canonical (German
+   *  grid feed); `energy-charts` is the fallback when SMARD is
+   *  unreachable. */
+  dataSource: "smard" | "energy-charts";
   // Production (MW)
   windOnshoreMw: number;
   windOffshoreMw: number;