Stop battery EcoQoS from starving frame capture during recording

Confirmed cause: on battery in a power-saving plan, Windows applies Power
Throttling (EcoQoS) to background work. StepForge records with its window
hidden, so the frame-capture worker renderer (plus the GPU and screen-capture
utility processes feeding it) get CPU-throttled. The throttled worker can't
sample the screen fast enough, so every click finds no fresh pre-click frame
and falls back to a slow post-click fresh shot — producing out-of-order,
dropped steps. It only reproduced on battery; plugged in (where EcoQoS stays
off even in eco mode) it worked. The capture log showed every click detected
but every one reporting "no frame qualified", with a ~4.6s stream startup.

Fixes:
- Chromium switches (disable-background-timer-throttling,
  disable-renderer-backgrounding, disable-backgrounding-occluded-windows) so
  Chromium stops de-prioritising and timer-throttling the hidden worker.
- New app/win-power.js opts the OS processes out of EcoQoS via
  SetProcessInformation(ProcessPowerThrottling, EXECUTION_SPEED off) and
  raises them to high priority. Applied to all live Electron processes when a
  session starts/resumes, and to the worker renderer the moment it is created
  (before it begins streaming). Best-effort, no-op off Windows.

The earlier mouse-hook EcoQoS opt-out already fixed click *detection*; this
fixes the frame-capture side that the same throttling was breaking.

app/stream-backend.js

@@ -340,6 +340,14 @@ async function createElectronHost(onEvent) {
     if (!win.isDestroyed()) win.destroy();
     throw err;
   }
+  // The worker is a hidden renderer, so on battery Windows would EcoQoS-throttle
+  // it and starve frame sampling. Clear that on its own OS process before it
+  // starts streaming. Best-effort; no-op off Windows.
+  try {
+    // eslint-disable-next-line global-require
+    const { keepProcessesResponsive } = require('./win-power');
+    if (!win.isDestroyed()) keepProcessesResponsive([win.webContents.getOSProcessId()]);
+  } catch { /* throttling tweak is optional */ }
   return {
     send(msg) {
       if (!win.isDestroyed()) win.webContents.send('capture-worker:command', msg);