9:16 recording: positionable crop strip + on-screen guide frame

New controls in the Registra tab: a 'Posizione taglio' slider choosing WHICH
strip of the screen survives the cover-crop when the format differs from the
display (ffmpeg crop x/y driven by cropX 0..1), and a guide checkbox. The
guide is a dashed frame + dimmed surroundings drawn on the output window
(outside #stage, last in DOM, overlay only — composeFrame never captures
it), live-updated from slider/format and hidden when no crop applies.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XVMQ2JhsJ77pGKqcvvFHqc

src/style.css

@@ -944,6 +944,18 @@ kbd {
   padding: 0 3px;
 }
 
+/* Recording crop guide on the output (overlay only, never recorded) */
+#rec-guide {
+  position: fixed;
+  display: none;
+  border: 2px dashed rgba(130, 185, 255, 0.85);
+  box-sizing: border-box;
+  /* dim everything outside the recorded area */
+  box-shadow: 0 0 0 9999px rgba(0, 0, 0, 0.45);
+  pointer-events: none;
+}
+#rec-guide.show { display: block; }
+
 /* Multi-output rows (tab Schermo) */
 .aux-row {
   display: flex;