feat: add remember password function

[bittoby]

Related Issue

Closes #1316

Description

Adds secure "Remember Me" credential storage for the login page using Electron's built-in safeStorage API, which encrypts credentials via OS-level keychain (macOS Keychain, Windows DPAPI, Linux Secret Service). No new dependencies required.

What it does:

• Encrypts and persists email/password credentials locally at ~/.eigent/saved_credentials.enc (file permissions 0o600)
• Auto-fills the most recently used account on login page load
• Shows a saved accounts dropdown (with masked emails) when focusing the email field, allowing quick account switching
• Each saved account can be individually removed from the dropdown
• "Remember Me" checkbox controls whether credentials are saved or cleared after successful login

Files changed:

• electron/main/index.ts — 3 IPC handlers (credentials-save, credentials-load, credentials-remove) using safeStorage.encryptString() / decryptString()
• electron/preload/index.ts — Exposed credential APIs to renderer via electronAPI bridge
• src/types/electron.d.ts — TypeScript declarations for new credential methods
• src/pages/Login.tsx — Remember Me checkbox, saved accounts dropdown with outside-click dismiss, auto-fill on mount
• 11 i18n locale files — Added remember-me, saved-accounts, remove-account keys (en, zh-Hans, zh-Hant, ja, ko, de, fr, es, it, ru, ar)

Testing Evidence (REQUIRED)

login.mp4

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline

[bittoby]

@Pakchoioioi @Wendong-Fan Could you please review the PR?
thank you

[Wendong-Fan]

thanks @bittoby ! could @4pmtong and @fengju0213 help reviewing this feature?