fix(strommix): per-field fallback to Energy-Charts when SMARD value missing

The widget rendered "Keine gültigen Live-Werte" the moment SMARD's
load filter (id 410) lagged behind the production filters by even one
slot — the aggregator coalesced "filter unavailable" with "filter
returned 0" and the percent calculation refused to divide by zero.

Three changes:

1. SmardSnapshot.values now uses `number | null` so callers can tell
   "filter genuinely missing" apart from "filter reported zero" (e.g.
   solar at night, decommissioned nuclear).
2. Aggregator pulls Energy-Charts (CMS strommix_de) in parallel and
   uses it as a per-field fallback for any SMARD null. New
   `dataSource: "smard+energy-charts"` flag travels in the response so
   the attribution stays honest. Pure SMARD when everything resolves;
   pure Energy-Charts when SMARD is entirely down; the hybrid label
   for the common case where only load drops out.
3. Widget renders the production breakdown + context row + price row
   even when the percentage is unavailable, with a clear "Lastwert
   von SMARD gerade nicht verfügbar" hint instead of going blank.

src/lib/strommix.ts

@@ -11,10 +11,13 @@ 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";
+  /** Where production + load came from. `smard` = pure SMARD (canonical);
+   *  `smard+energy-charts` = SMARD with Energy-Charts as per-field
+   *  fallback for filters SMARD couldn't resolve at the snapshot's slot
+   *  (typically the load filter when SMARD's pipeline lags more than
+   *  the production filters); `energy-charts` = SMARD entirely
+   *  unreachable, full fallback. */
+  dataSource: "smard" | "smard+energy-charts" | "energy-charts";
   // Production (MW)
   windOnshoreMw: number;
   windOffshoreMw: number;