Fix Sites fallback file placeholder

[HarshMN2345]

Summary

• keep empty Sites fallback file values empty in build settings
• leave index.html as placeholder-only unless the user explicitly sets it or changes framework/adapter defaults

Tests

• bun run check

[greptile-apps]

Greptile Summary

This PR keeps empty Sites fallback-file values empty while retaining framework and adapter defaults for explicit selection changes.

• Normalizes a missing fallback value to an empty form value.
• Removes the reactive behavior that replaced empty static fallback values with the adapter default.

Confidence Score: 5/5

The PR appears safe to merge with no actionable changed-code defects identified.

The removed reactive defaulting duplicated the existing explicit framework and adapter transition logic and prevented empty fallback values from remaining empty; the retained transitions still apply defaults when selections change.

Important Files Changed

Filename	Overview
src/routes/(console)/project-[region]-[project]/sites/site-[site]/settings/updateBuildSettings.svelte	Empty fallback values now remain placeholder-only, while framework and adapter transitions continue to populate their configured defaults.

_{Reviews (1): Last reviewed commit: "fix sites fallback placeholder" | Re-trigger Greptile}