feat: add source control action to abort merge

[leynier]

Summary

• add a Source Control dropdown action to abort an in-progress merge
• route git merge --abort through local, SSH, runtime RPC, preload, and renderer git clients
• add a confirmation dialog and refresh the affected worktree status after aborting

Closes #2090

Validation

• pnpm exec vitest run --config config/vitest.config.ts src/main/git/status.test.ts src/main/providers/ssh-git-provider-merge.test.ts src/main/ipc/filesystem.test.ts src/main/runtime/rpc/methods/git.test.ts src/renderer/src/runtime/runtime-git-client-merge.test.ts src/renderer/src/components/right-sidebar/source-control-dropdown-items.test.ts
• pnpm run typecheck:node && pnpm run typecheck:web
• git diff --check

Note: validation emits the existing Node engine warning because this shell is using Node v26.0.0 while the repo requests Node 24.

[brennanb2025]

Thanks for this PR, and sorry for the delay in reviewing it!

Reviewed and pushed a follow-up commit: d4587f5fc review: make abort merge reachable during conflicts.

I rebased the PR onto current main and decided to keep this PR over #2423. The main issue I found after rebase was that current Source Control hides the commit area while a merge/conflict operation is active, so a dropdown-only Abort merge action could be unreachable exactly when users need it. The follow-up surfaces Abort merge directly in the conflict summary and merge operation banner.