fix(settings): saving the Settings page reverts backend coverage propagation and an empty coverage field saves as 0 #522

Summary

After #500 the backend copies a changed default_coverage to service_configs. Saving the Settings page then undoes that propagation. Separately, an empty global coverage field saves as 0.

Location (origin/main 496d9d7)

• frontend/src/settings.ts:3560 saveGlobalSettings PUTs every service with base?.coverage ?? settings.default_coverage, or the SP card's DOM value (3562-3566).
• internal/api/handler_config.go:205 updateServiceConfig overlays existing.Coverage = cfg.Coverage unconditionally, so the stale value wins.
• No coverage confirm dialog exists; only term and payment have one.
• frontend/src/settings.ts:3482 Number('') is 0, and the check at 3483 accepts 0. wireInlineRangeValidation (:247) also accepts the empty string. Same pattern in frontend/src/plans.ts:1345.

Failure scenario

Probe: global coverage 50, ec2 and savings-plans-compute rows at 50. Set global to 0 and Save. Result: global 0, ec2 50, savings-plans-compute 50. Clearing the global coverage field and saving stores 0.

Context

Found in the #510 review. Related: #135 (closed), #225 (closed, same class for term/payment), #345.

Acceptance

Saving a changed global coverage leaves service rows at the propagated value, and an empty coverage field is rejected with an inline error instead of saving 0.