fix(nix): build desktop package with electron

[tommoa]

The desktop package no longer has the Tauri src-tauri tree, so the Nix desktop derivation packages the Electron app instead of using cargo-tauri. The Electron renderer and main outputs are built with the shared node_modules derivation, and the unpacked app uses Nix-provided Electron to avoid sandbox downloads.

Issue for this PR

Closes #26120

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR modifies the desktop flake to use the new electron build process for the desktop app. There were some things that needed to be modified to have parity with the old flake:

• nix does not allow writable locations, so the auto-updater needed to be disabled
• electron-builder doesn't generate a .desktop for Linux, so a .desktop is made in the flake

How did you verify your code works?

nix run .#desktop on macOS 26.4.1, which worked.

# nix run .#desktop
(node:65828) ExperimentalWarning: SQLite is an experimental feature and might change at any time
(Use `OpenCode --trace-warnings ...` to show where the warning was created)
17:31:20.631 › app starting { version: '1.14.40+03c4c47', packaged: true, onboardingTest: false }
17:31:20.806 › tauri migration: already done, skipping
17:31:20.868 › sidecar connection started { url: 'http://127.0.0.1:65516' }
17:31:20.869 › spawning sidecar { url: 'http://127.0.0.1:65516' }
[server] Loaded shell environment with -il (69 vars)
17:31:22.045 › loading task finished
17:31:22.046 › init step { step: { phase: 'done' } }
17:31:22.550 › awaiting server ready
17:31:22.551 › server ready { url: 'http://127.0.0.1:65516' }

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR