fix(desktop): keep host seeds in the clear, and keep the ones we cannot read

[kamrul1157024]

What broke

Installing a new desktop build looked like the app had forgotten every host it was paired with.

Each host's device seed went through Electron's safeStorage. On macOS that is the Keychain, and the Keychain binds an item to the code signature that wrote it. electron-builder.yml sets identity: null, so every build is ad-hoc signed with a fresh identity — the new build could not read what the old one wrote.

From there the loss was automatic:

1. load() skipped any host whose seed would not decrypt (if (!seed) continue).
2. persist() rebuilds the file from what is in memory.
3. So the first pair or rename after the upgrade erased the skipped hosts from hosts.json for good.

What changed

• Nothing is encrypted. The seed authenticates to a daemon on loopback or a tailnet, sits beside a plaintext helios.db, and hosts.json is already 0600 — the Keychain was buying very little and cost real data. safeStorage is gone from the file.
• An entry left over from the encrypting builds is kept verbatim through a write rather than dropped. It is unreadable, but unreadable is not a reason to delete someone's host.

Anyone upgrading past this keeps their old entries in the file; a host that was encrypted still has to be paired again, because no key for it exists on the machine any more.

Test plan

• desktop/e2e/hosts-store.spec.ts — seeds a readable host beside a legacy encrypted: true one, renames through the Hosts pane to force a write, asserts the legacy entry survived and the readable one is written back with no encrypted flag. Fails on main, passes here.
• npm test — 292 pass
• npx playwright test — 58 pass
• npm run typecheck