fix: auto-register Linux .desktop file with %u to unblock OAuth callback

[Rene0422]

Related Issue

Closes #1525

Description

Fixes the Linux auth hang where login stalls at "Logging in......." after the OAuth provider redirects back via eigent://auth/callback?token=....

Root cause. On Linux, app.setAsDefaultProtocolClient only invokes xdg-mime default against a .desktop file that must already exist — it does not create one. AppImage builds ship no such file, and users who author their own (as the reporter did) frequently omit the %u/%U field code. Without %u, xdg-open drops the eigent:// URL before launching the app. The running instance's second-instance event then fires with a URL-less argv:

[info] second-instance [
'/opt/eigent-0.0.89/bin/squashfs-root/eigent',
'--allow-file-access-from-files',
... // no eigent:// URL here
]

So handleProtocolUrl is never called and the token never reaches the renderer.

Fix.

• New electron/main/utils/linuxProtocol.ts — on every packaged Linux launch, writes a self-managed eigent-protocol-handler.desktop into ~/.local/share/applications/ with Exec="<path>" %u and MimeType=x-scheme-handler/eigent;, then refreshes update-desktop-database and xdg-mime default. Prefers $APPIMAGE so the registration survives AppImage version bumps; falls back to process.execPath. Skipped in dev mode since the existing dev branch of setupProtocolHandlers handles that case.
• electron/main/index.ts — wires the helper into setupProtocolHandlers, and re-runs registration if second-instance fires on Linux without a URL so stale desktop files self-heal on the next login attempt.
• electron-builder.json — adds MimeType=x-scheme-handler/eigent; to the AppImage's bundled .desktop entry so appimaged-managed installs pick up the association at install time.

Testing Evidence (REQUIRED)

• 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.

Verification performed:

• npm run type-check passes (exit 0).
• npm run dev produces clean Vite bundles for dist-electron/main and dist-electron/preload.
• Manual: on Fedora 43 / XFCE4 reproducing the reporter's setup, the app now writes ~/.local/share/applications/eigent-protocol-handler.desktop with Exec="<exec>" %u. Triggering xdg-open 'eigent://auth/callback?token=test' against the running app delivers the URL into the second-instance argv and handleProtocolUrl fires auth-token-received, completing login.

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