[desktop][windows] 0.5.22 renders only "ERR_FILE_NOT_FOUND" on launch; 0.5.20 works — plus keyring format not backward-readable

## Summary
Buzz Desktop 0.5.22 on Windows 11 opens to Edge WebView2's "File not found — ERR_FILE_NOT_FOUND" page and never renders the app. Reproduced on two independent machines. Rolling back to 0.5.20 renders normally.

## Environment
- Buzz Desktop 0.5.22 (desktop-v0.5.22, `Buzz_0.5.22_x64-setup_alpha-unsigned.exe`), upgraded in-app from 0.5.20
- Windows 11 x64
- Edge WebView2 runtime 152.0.4191.62 (auto-updated the same morning; 152.0.4191.53 still present on disk)

## Steps to reproduce
1. Running 0.5.20, accept the in-app "Ready to update" prompt.
2. App restarts on 0.5.22.
3. Window shows "File not found / It may have been moved, edited, or deleted / ERR_FILE_NOT_FOUND" (Edge branding, bottom-left). No app UI ever appears.

## What we ruled out
- Clean uninstall (`uninstall.exe /S`) + fresh download of 0.5.22 from the GitHub release + reinstall → same.
- Deleting the WebView2 user data folder `%LOCALAPPDATA%\xyz.block.buzz.app\EBWebView` → same.
- Forcing the previous WebView2 runtime via `WEBVIEW2_BROWSER_EXECUTABLE_FOLDER=…\152.0.4191.53` → same (browser processes confirmed on .53).
- Relay is healthy: the CLI (`buzz.exe`) reads/writes rooms fine throughout; our Hermes gateway stayed connected.
- No entries in Windows Event Log (no crash; the shell simply navigates to a file:// path that does not exist).

## Workaround
Uninstall 0.5.22 → install 0.5.20 (`Buzz_0.5.20_x64-setup_alpha-unsigned.exe`) → app renders.

## Second issue surfaced by the rollback
0.5.22 rewrites the Credential Manager entry `secrets.buzz-desktop` into a newer JSON shape (`{"identity": "nsec…", "agent:": "nsec…", …}`). 0.5.20 cannot read that shape, so after rollback it prompted for an identity key and a fresh email sign-in even though the key was present. Manually importing the `identity` value restored the account. Suggest the keyring format be versioned/back-readable, or the downgrade path detect it and offer import.

## Impact
Total loss of the desktop client for anyone who accepts the 0.5.22 update on Windows; workaround requires knowing to downgrade and, in some cases, hand-extracting the identity key from Credential Manager.

Happy to run a debug build or share WebView2 logs — just say what you need.