feat(recs): plumb provider SavingsPercentage to GUI (CLI/GUI parity; AWS missing on_demand_cost shows '—') #1106

Gap

The CLI/reporter shows AWS's authoritative rec.SavingsPercentage directly (internal/reporter/reporter.go:45, sourced from AWS EstimatedMonthlySavingsPercentage in parser_ri.go; the Azure and GCP converters also set SavingsPercentage). But SavingsPercentage is dropped at the API boundary -- it was never on config.RecommendationRecord, so it is never serialized to the frontend.

Because of that, the GUI re-derives the effective savings % client-side via effectiveSavingsPct(savings, on_demand_cost, ...). That recomputation:

1. can drift from the provider's authoritative number, and
2. returns null / "—" when on_demand_cost is missing even though the provider gave a real % (the fix(frontend/recs): return null from effectiveSavingsPct on AWS reconstruction fallback (followup to #321) #323 case).

The CLI never has these problems because it uses the provider value directly. This is a CLI-vs-GUI parity gap on a money-path display.

Fix (full-stack)

• Backend struct (internal/config/types.go): add nullable RecommendationRecord.SavingsPercentage *float64 (nil = provider did not report a %). Persisted via the recommendations row's JSONB payload column -- no DDL/migration needed (recs are stored as JSONB; same as OnDemandCost). Absent represented as nil, never 0.
• Mapping (internal/scheduler/scheduler.go convertRecommendations, the only builder mapping a common rec to a record): set SavingsPercentage: nonZeroPtr(rec.SavingsPercentage), parity with OnDemandCost. A provider 0 round-trips as nil.
• Frontend type (frontend/src/types.ts): add savings_percentage?: number | null to LocalRecommendation.
• Frontend display/sort (frontend/src/recommendations.ts): the effective-% column and its sort accessor prefer r.savings_percentage when it is a finite, non-null number; fall back to effectiveSavingsPct(r) otherwise. Net effect: GUI shows the same number the CLI shows, and AWS recs missing on_demand_cost show the provider % instead of "—".

Scope

Full-stack (Go struct + scheduler mapping + frontend type + display/sort), implemented in PR #1104 alongside the net-savings fix.

Related: #323, #1035.