fix(desktop): allow builds from non-git checkouts with fallback commit

[allin2]

Summary

The desktop build script threw a fatal error when the project source was not a git repository.

Motivation

Fixes #50823

Users who downloaded the source as a ZIP archive or deployed via bootstrap installer without a .git directory could not build the Windows desktop client.

Changes

• fix: Added fromFallback() that provides a placeholder commit hash for non-git builds
• The desktop app treats 0000000... as 'unknown' and skips commit-pinned first-launch bootstrap
• Added warning message when fallback is used

Validation

• Non-git builds now complete successfully with a placeholder commit
• Warning message informs users about the limitation
• Production builds from git checkouts still work as before

Checklist

• Code follows the project's style guidelines
• No unrelated changes included
• Commit message follows Conventional Commits format

[teknium1]

Thanks for addressing the non-git desktop-build failure. The premise remains valid on current main: apps/desktop/scripts/write-build-stamp.mjs:81-92 still exits when neither CI metadata nor a git checkout is available, and the linked #61657 reports the same Windows failure.

Problems

• The submitted files are the pre-migration CJS paths. Current main renamed them in 39d09453f95e8aefc0c97e5d9b30ff341cae9ed8 to bootstrap-runner.ts, bootstrap-runner.test.ts, and write-build-stamp.mjs, so the patch needs a port to the current desktop implementation.
• Current main's runner has newer fresh-install pin control at apps/desktop/electron/bootstrap-runner.ts:548-574. The port must keep an unknown fallback out of -Commit/--commit while retaining that existing-checkout behavior.
• The PR adds runner tests but no regression test for the failing stamp-generator path (write-build-stamp.mjs:81-92) without git or GITHUB_SHA.

Suggested changes

• Port the fallback stamp and branch-based bootstrap resolution to the TS/MJS files, preserving immutable pins for CI/git builds and an explicit unpinned warning for fallback builds.
• Add an isolated no-git generator test plus fallback bootstrap/pin-argument coverage.

Automated hermes-sweeper review.