fix(web): preserve untouched metadata levels when toggling a provider (#226)

toggleLevelProvider merged the toggled level into the levelChains state
object, which starts empty. After the first toggle the state collapsed to
a single level, so once chainDirty flipped true activeLevelChains dropped
every untouched level and the dialog rendered them empty.

Seed from the full resolved activeLevelChains map and write it back the
same way reorderLevel already does, so disabling a source on one TV level
no longer blanks the others on the first toggle.

The earlier redesign (b9a9613) only patched the per-level source fallback,
which kept the toggled level's items but did not preserve the rest of the
map; builds containing it still reproduced the issue.

Part of #213

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

web/src/components/admin/libraries/useLibraryForm.ts

@@ -257,12 +257,10 @@ export function useLibraryForm({
   }
 
   function toggleLevelProvider(level: string, index: number) {
-    setLevelChains((prev) => {
-      const source = prev[level] ?? activeLevelChains[level] ?? [];
-      const updated = [...source];
-      updated[index] = { ...updated[index]!, enabled: !updated[index]!.enabled };
-      return { ...prev, [level]: updated };
-    });
+    const source = activeLevelChains[level] ?? [];
+    const updated = [...source];
+    updated[index] = { ...updated[index]!, enabled: !updated[index]!.enabled };
+    setLevelChains({ ...activeLevelChains, [level]: updated });
     setChainDirty(true);
   }