feat(P6/T05): cross-framework annotation launcher JS widget

ihf-annotation-launcher.js: vanilla JS, no framework dependency. Scans DOM for
data-widget-id elements, injects Annotate trigger, opens inline form, POSTs to
/widgets/:widgetId/annotations. Works in React/Vue-rendered pages via
MutationObserver. Feature-gated by IHP_ANNOTATION_LAUNCHER=true env var
(Config.hs AnnotationLauncherEnabled, FrontController layout conditional).
Docs: docs/annotation-launcher.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Config/Config.hs

@@ -3,9 +3,13 @@ module Config where
 import IHP.Prelude
 import IHP.Environment
 import IHP.FrameworkConfig
+import System.Environment (lookupEnv)
+
+-- | Feature flag: set IHP_ANNOTATION_LAUNCHER=true to inject the JS launcher.
+newtype AnnotationLauncherEnabled = AnnotationLauncherEnabled Bool deriving (Typeable)
 
 config :: ConfigBuilder
 config = do
-    -- See https://ihp.digitallyinduced.com/Guide/config.html
-    -- for what you can do here
-    pure ()
+    launcherEnv <- liftIO (lookupEnv "IHP_ANNOTATION_LAUNCHER")
+    option (AnnotationLauncherEnabled (launcherEnv == Just "true"))
+    pure ()