fix: stop returning private key to renderer on export

[0x-SquidSol]

Summary

• The wallet:export-private-key IPC handler was returning the raw private key string to the renderer process after already copying it to the clipboard
• This unnecessarily exposed key material across the IPC bridge where it could be captured by compromised renderer code or DevTools
• The handler now returns a success flag; the renderer shows a clipboard confirmation message instead of the plaintext key

Test plan

• Export a private key — verify it copies to clipboard and shows confirmation message
• Verify clipboard auto-clears after 30 seconds
• Confirm key is no longer visible in DevTools Network/IPC inspection
• Run pnpm run typecheck — passes clean
• Run pnpm run test — all wallet tests pass (19/19)

[nullxnothing]

The runtime behavior here makes sense, but the renderer contract also needs to be updated. wallet:export-private-key would now return rue, while src/types/daemon.d.ts still declares �xportPrivateKey: (walletId: string) => Promise<IpcResponse>. Please update the preload/type surface so the IPC contract matches the new behavior.

[nullxnothing]

Closed as superseded by #75, which reapplies the corrected security hardening on top of current main. This avoids merging stale/conflicted branches and preserves the fixes with passing CI/CodeQL.