feat: Windows download link + self-served update feed + install tracking

Downloads/updates:
- config: DOWNLOADS_DIR (override to a mounted volume in prod).
- static.js: serves /downloads/* (installer, latest.yml, .blockmap) with range support for
  resumable + differential auto-updates; /download/windows redirects to the current .exe
  (stable link). Landing page gets a "Download the Windows desktop app" button (hidden in-app).

Install tracking (who installed the app):
- db app_installs + repos.appInstalls (upsert by install_id, fills in the user on sign-in).
- POST /api/v1/telemetry/install (records install + user once authenticated);
  GET /api/v1/admin/installs (admin: list installs with user/version/os/last-seen).
- desktop main.js: stable per-install id in userData, exposed via preload
  (bizConnectNative.installId/version/os); home.html reportInstall() posts it after login.
- e2e: +2 checks (telemetry recorded, admin sees it). 119/119. build batch20.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/main.js

@@ -10,6 +10,25 @@
 // Server origin is configurable so the same build works against prod or a dev server.
 const { app, BrowserWindow, session, desktopCapturer, shell, Menu, ipcMain, nativeImage } = require('electron');
 const path = require('path');
+const fs = require('fs');
+const os = require('os');
+const crypto = require('crypto');
+
+// A stable per-install id (persisted in userData), so the server can count installs and
+// associate them with the user who signs in. Created once, then reused across launches.
+function getInstallId() {
+  try {
+    const p = path.join(app.getPath('userData'), 'install-id');
+    if (fs.existsSync(p)) { const v = fs.readFileSync(p, 'utf8').trim(); if (v) return v; }
+    const id = crypto.randomUUID();
+    fs.writeFileSync(p, id);
+    return id;
+  } catch (_) { return 'unknown'; }
+}
+// The renderer (web app) reads this synchronously to report telemetry after login.
+ipcMain.on('get-install-info', (e) => {
+  e.returnValue = { installId: getInstallId(), appVersion: app.getVersion(), os: process.platform + ' ' + os.release() };
+});
 // Auto-update: only NATIVE shell changes (this .exe) need this — all web/UI changes arrive
 // live from the server. Checks the self-hosted feed (publish config in package.json →
 // https://remote.bizgaze.com/downloads/latest.yml), downloads in the background, and installs