[AI] Make rebuild-electron actually rebuild better-sqlite3

PR #7712 simplified rebuild-electron to just `electron-rebuild -f -o
better-sqlite3,bcrypt` from the repo root. Two problems with that:

  1. Without `-m`, electron-rebuild scans the root workspace's package.json
     for native deps. better-sqlite3 isn't a direct root dep — it lives
     under packages/sync-server/ — so the scan returns no candidates and
     the rebuild silently no-ops.
  2. Without --build-from-source, electron-rebuild defers to
     prebuild-install, which downloads a stale prebuilt binary keyed off
     better-sqlite3's package.json (ABI 127) instead of recompiling
     against Electron 39's bundled Node ABI 140. The download succeeds
     and "Rebuild Complete" prints, but the resulting `better_sqlite3.node`
     can't `dlopen` inside Electron's utility process — sync-server
     crashes immediately on db init, the renderer's startSyncServer IPC
     never resolves, and the e2e test hangs on "Configure your server".

Point -m at packages/desktop-electron (which transitively pulls in
better-sqlite3 and bcrypt via @actual-app/sync-server) and force a real
compile via --build-from-source. Verified locally: better-sqlite3
rebuilds to darwin-arm64-140 and the desktop e2e onboarding test passes
in 6s instead of hanging for 60s.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -52,7 +52,7 @@
     "playwright": "yarn workspace @actual-app/web run playwright",
     "vrt": "yarn workspace @actual-app/web run vrt",
     "vrt:docker": "./bin/run-vrt",
-    "rebuild-electron": "./node_modules/.bin/electron-rebuild -f -o better-sqlite3,bcrypt",
+    "rebuild-electron": "./node_modules/.bin/electron-rebuild -m ./packages/desktop-electron -o better-sqlite3,bcrypt --build-from-source -f",
     "rebuild-node": "yarn workspace @actual-app/core rebuild",
     "lint": "oxfmt --check . && oxlint --type-aware --quiet",
     "lint:fix": "oxfmt . && oxlint --fix --type-aware --quiet",