Let effects declare whether they need source images

Whether an effect works from the loaded photographs is now part of the
declarative plug-in contract, next to params(): Effect::needsSources()
defaults to true, and blur — which only softens what it is handed — says
no and blurs an empty canvas when given nothing.

Only the first stage of a cascade ever sees the sources, so
chainNeedsSources() just asks the first stage. Both front-ends gate on
that instead of on "are there any images": the CLI only refuses an empty
batch when the chain wants one, and the interface renders a blur-led
chain with no images loaded, saying whichever thing is actually missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/chain.cpp

@@ -26,6 +26,11 @@ Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image
     return image;
 }
 
+bool chainNeedsSources(const std::vector<ChainStage>& stages) {
+    if (stages.empty() || !stages.front().effect) return false;
+    return stages.front().effect->needsSources();
+}
+
 std::string chainTitle(const std::vector<ChainStage>& stages) {
     std::string text;
     for (const auto& stage : stages) {