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/main.cpp

@@ -40,7 +40,7 @@ int renderHeadless(const effector::Options& options) {
     for (const auto& failure : failures) {
         std::cerr << "effector: skipping " << failure.path << ": " << failure.message << "\n";
     }
-    if (sources.empty()) {
+    if (sources.empty() && effector::chainNeedsSources(stages)) {
         std::cerr << "effector: no usable source images\n";
         return 2;
     }