ci: setup signtool and fix Windows code signing in release workflow

[Radhikaa-chauhan]

Fixes CircuitVerse#636

Describe the changes you have made in this PR -

• Added Windows code signing to the Tauri desktop release workflow.
• Securely loads the signing certificate from GitHub Secrets and signs the Windows executable during CI.
• Applies signing only to Windows builds, without affecting macOS or Linux releases.

Screenshots of the UI changes (If any) -

N.A.

Code Understanding and AI Usage

Did you use AI assistance (ChatGPT, Claude, Copilot, etc.) to write any part of this code?

• No, I wrote all the code myself
• Yes, I used AI assistance (continue below)

Explain your implementation approach:

• This change adds automated Windows code signing to the desktop release workflow.
• The signing certificate is securely loaded from GitHub Secrets and used at runtime to sign the Windows executable using signtool.
• Signing is limited to Windows builds to keep other platforms unaffected.

---

Checklist before requesting a review

• I have added proper PR title and linked to the issue
• I have performed a self-review of my code
• I can explain the purpose of every function, class, and logic block I added
• I understand why my changes work and have tested them thoroughly
• I have considered potential edge cases and how my code handles them
• If it is a core feature, I have added thorough tests
• My code follows the project's style guidelines and conventions

---

Note: Please check Allow edits from maintainers if you would like us to assist in the PR.