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/components/blocks/StrommixBlock.svelte

@@ -186,18 +186,25 @@
       </div>
     {:else if !live}
       <div class="p-6 text-center text-neutral-500 text-sm">Lade Live-Daten…</div>
-    {:else if renewablePct == null}
-      <div class="p-6 text-sm text-amber-700">Keine gültigen Live-Werte verfügbar.</div>
     {:else}
       <div class="p-6 text-center">
-        <div class="text-6xl sm:text-7xl font-extrabold tabular-nums {accentClasses.pct}">
-          {Math.round(renewablePct)}%
-        </div>
-        <div class="mt-1 text-sm text-neutral-700">Wind &amp; Solar am Bedarf</div>
-        <div class="mt-3 text-base font-semibold {accentClasses.label}">→ {statusLabel}</div>
-        {#if argumentHtml}
-          <div class="prose prose-sm mt-3 max-w-prose mx-auto text-neutral-800">
-            {@html argumentHtml}
+        {#if renewablePct != null}
+          <div class="text-6xl sm:text-7xl font-extrabold tabular-nums {accentClasses.pct}">
+            {Math.round(renewablePct)}%
+          </div>
+          <div class="mt-1 text-sm text-neutral-700">Wind &amp; Solar am Bedarf</div>
+          <div class="mt-3 text-base font-semibold {accentClasses.label}">→ {statusLabel}</div>
+          {#if argumentHtml}
+            <div class="prose prose-sm mt-3 max-w-prose mx-auto text-neutral-800">
+              {@html argumentHtml}
+            </div>
+          {/if}
+        {:else}
+          <!-- Production data arrived but load is missing → no percentage,
+               but we still show what we have so the widget isn't blank. -->
+          <div class="text-sm text-amber-700">
+            Lastwert von SMARD gerade nicht verfügbar — Prozentanzeige ausgesetzt.
+            Erzeugung wird unten in der Detailansicht weiter angezeigt.
           </div>
         {/if}
       </div>
@@ -270,9 +277,14 @@
         <div class="text-neutral-500 font-semibold">Last</div><div class="font-semibold">{fmtMw(live.loadMw)}</div>
       </div>
       <p class="mt-1 text-[11px] text-neutral-500">
-        Quelle: {live.dataSource === "smard"
-          ? "smard.de (Bundesnetzagentur)"
-          : "energy-charts.info (ENTSO-E + SMARD)"}
+        Quelle:
+        {#if live.dataSource === "smard"}
+          smard.de (Bundesnetzagentur)
+        {:else if live.dataSource === "smard+energy-charts"}
+          smard.de + energy-charts.info (Last-Wert ergänzt)
+        {:else}
+          energy-charts.info (ENTSO-E + SMARD)
+        {/if}
         — 15-Min-Auflösung, Stand:&nbsp;{fmtTime(live.measuredAtUnix)}&nbsp;Uhr.
         Konventionell = Braunkohle + Steinkohle + Erdgas + Kernkraft (Pumpspeicher,
         Sonstige Konventionelle und Wasserkraft sind separat ausgewiesen).