Desktop update install: the leave guard after a reload or a lost event #357

Follow-ups to the unsaved-edits guard around a remote update install in #349. Found by reading the code.

• A renderer reload during an allowed install forgets the block. updateInstall is a module variable in apps/web/src/components/files/unsavedFileEdits.ts, so the Files block is dropped and saving runs again while the install may still restart the app.
• If the desktop:update-install-ended event is missed or the allow reply is rejected, the "Installing an update" dialog (UnsavedChangesCloseDialog.tsx) stays up with no way out. Add one, such as asking the desktop for the install state again.
• A Files source registered during the install is not held. blockForUpdateInstall holds a snapshot of the sources, and registerUnsavedSource adds a source without taking a hold.
• Nothing ties the event name in apps/web/src/tauriDesktopBridge.ts to the one in apps/desktop/src-tauri/src/remote_update_delegate.rs. Add a test or a shared constant.
• Not decided: whether the always-open installing dialog makes the Manage devices toast unreachable.