Never take a post-click screenshot when a pre-click frame exists

The remaining 'captured slightly after the click' reports came from the
fresh-shot fallback, which grabs the screen when the click is processed
(after it). The previous lead change made that fallback *more* likely: a
frame now had to be >=120ms before the click to qualify, so on machines
where the capture stream can't always keep a frame that old buffered, more
clicks fell through to the post-click shot.

Make the click-lead a two-tier preference instead of a hard gate in
selectFrameForClick:
1. newest frame captured at least leadMs before the click (ideal margin), else
2. newest frame captured before the click at all.
Only when no pre-click frame exists does the caller fresh-shot. leadMs is
threaded through the stream backend to the worker so both selection paths
agree. Verified end to end: frames land ~120-170ms before each click,
markers stay at 0.00%, and the 8-click burst still saves all 8.

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

app/stream-backend.js

@@ -210,7 +210,7 @@ class StreamCaptureBackend {
    * Resolves null when no frame qualifies (caller falls back) — and also on
    * timeout, which additionally counts toward unhealthiness.
    */
-  frameForClick({ clickPos = null, clickAt = Date.now(), strict = true } = {}) {
+  frameForClick({ clickPos = null, clickAt = Date.now(), strict = true, leadMs = 0 } = {}) {
     if (!this.active || !this.host) return Promise.resolve(null);
     const displays = [...this.streams.values()].filter((s) => s.ready).map((s) => s.display);
     const display = clickPos ? displayForDipPoint(clickPos, displays) : (displays[0] || null);
@@ -234,6 +234,7 @@ class StreamCaptureBackend {
         displayId: display.id,
         clickAt,
         strict,
+        leadMs,
       });
     });
   }