watched: single DONE_RATIO source constant replaces the seven hardcoded 0.95 literals

server/app.py DONE_RATIO is injected into index.html like {{VERSION}}, app.js
reads it as window.DONE_RATIO (fallback = same default). Change the threshold
in exactly one place now.

static/index.html

@@ -35,6 +35,9 @@
      route) with the git SHA at startup — a correct cache key per commit.
      No hand-bumping; JSV in app.js reads the same value from its own URL. -->
 <script src="hls.min.js"></script>
+<!-- Watched threshold: server injects its DONE_RATIO source constant here;
+     app.js reads it as DONE_RATIO. Server uses the same value for its stats. -->
+<script>window.DONE_RATIO = {{DONE_RATIO}};</script>
 <script src="app.js?v={{VERSION}}"></script>
 </body>
 </html>