fix(start-scripts): find bundled privacy_core.dll next to script

start.bat and start.sh only checked the source-tree DLL path
(``privacy-core/target/release/privacy_core.dll``), not the bundled
location where MSI/AppImage/DMG installers stage the library directly
next to the script in backend-runtime/.

Users running start.bat from inside an MSI install dir (a documented
workaround when the desktop shell crashes) saw a scary "install Rust"
warning even though the DLL was sitting right next to them. See issue
#319 for the user-reported confusion.

Fix: add a fallback check for the bundled location before falling
through to the "build privacy-core from source" warning. Source-tree
behavior unchanged — the source path is still preferred when present.

Also re-stamps the v0.9.81 source archive: ``release_digests.json``
v0.9.81 zip hash updated to point at the rebuilt source archive that
contains these script changes. MSI/EXE/sig hashes are unchanged (the
scripts live at the repo root, not inside the desktop bundle).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/data/release_digests.json

@@ -43,7 +43,7 @@
     "ShadowBroker_0.9.8_x64_en-US.msi": "fe22f9d51e4360d74c18a7250c2fbb9ed4fa4c7a884b3ac0d04a21115466386b"
   },
   "v0.9.81": {
-    "ShadowBroker_v0.9.81.zip": "42f8a51f9a5690d1e7349d90d8ecf2d163c9061d6cf90c69ee03647a785437ff",
+    "ShadowBroker_v0.9.81.zip": "31e5273253f329746ca2c4dc3f1da47e50a30981759b33056b3174e6474b9e4b",
     "ShadowBroker_0.9.81_x64-setup.exe": "eca884b9d37eeccd0f11c91dcc6f6ae1b3609d9dee72bd73c37c9a427babfef2",
     "ShadowBroker_0.9.81_x64_en-US.msi": "a45b177c26c95d2b28d71592d7147e88ff4e104865f214fde11249d311ec9e25"
   }