Report BUILDER_FALLBACK metric when falling back to local builder

[adityamaru]

When startBlacksmithBuilder() fails and falls back to a local builder, we now report a BUILDER_FALLBACK failure via reportBuildPushActionFailure() before logging the warning. This lets us track how often users experience fallbacks as a distinct event from the underlying root cause (e.g. BUILDER_STARTUP).

How it works:

• Adds BUILDER_FALLBACK to the type union in reportBuildPushActionFailure() in src/reporter.ts
• Calls the reporter in the catch block of startBlacksmithBuilder() in src/main.ts, after the nofallback check but before the warning log and return

The reporter already handles its own errors internally (catches and warns), so a failure to report the metric won't block the fallback path. The companion PR in useblacksmith/web adds BUILDER_FALLBACK to the backend validation rule and two new Grafana panels to the Docker Overview dashboard.

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