fix: kill discord before writing betterdiscord.asar

[Huderon]

Summary

fix for #426

Closes Discord before attempting to write betterdiscord.asar. Not sure why this wasn't an issue in the Electron version, but Discord definitely has a handle on the file.

Type of change

• Bug fix
• New feature / behavior change
• UI / component change (frontend)
• Backend / installer logic (Go)
• Build / release / CI
• Refactor (no behavior change)
• Documentation
• Other:

Testing

• bun run check passes (frontend, from frontend/)
• bun run lint passes (frontend, from frontend/)
• bun run test passes (frontend, from frontend/)
• go test ./... passes (backend)

AI disclosure

• I did not use AI assistance for this PR
• I used AI assistance and personally reviewed all generated output before submitting
• An AI agent generated part or all of this PR with minimal personal review. Details:

[zerebos]

@doggybootsy told you

[doggybootsy]

We should look into how electron writes asars, as this will have issues if the user selects stable but has ptb/canary open

[Huderon]

Looks like it has R/W share perms, so we should be able to write to it despite the handle