feat(billing): adopt subscription.* + usage.bars WS methods (issue #628) (#629)

* feat(billing): adopt subscription.* + usage.bars WS methods (issue #628)

Add Billing/Plan screen + WS client surface for the backend release audit
(hermes-agent 0bf44d557..614dc194e). credits.view was removed upstream;
subscription.{state,preview,change,resume,upgrade} + usage.bars replace it.

- WsMethods: add SUBSCRIPTION_* + USAGE_BARS constants
- BillingModels: @Serializable request/response (snake_case, ignoreUnknownKeys)
- BillingRepository: WS dispatch via HermesWsClient.request + typed decode;
  HermesRpcException (-32601) surfaces as featureUnavailable, not a crash
- BillingViewModel: plan/usage load + upgrade/change/resume/cancel actions
- BillingScreen: HermesScaffold composable (plan card, usage bars, actions)
- NavigationKeys + ScreenRegistry + strings (en/ko) wire BillingScreen into
  the INSPECT drawer section

Verified: ./gradlew ktlintCheck + compileDebugKotlin both BUILD SUCCESSFUL.

* fix(billing): align models + UI to live gateway wire shape (issue #628)

Live-probed the 6 subscription.* / usage.bars RPCs against the running
backend (hermes-agent dashboard :9119, 2026-07-19). Findings:

- subscription.state returns logged_in / current / tiers / usage (nested),
  NOT the legacy subscription_type_id / renews_at fields the models assumed.
- Mutating RPCs (preview/change/resume/upgrade) return ok:false *results*
  with error/message/payload (e.g. invalid_token when not portal-authed),
  not RPC-level errors — so the ViewModel's HermesRpcException guard never
  fired and the screen silently half-decoded null fields.
- subscription.upgrade also returns an idempotency_key not in the model.

Changes:
- BillingModels: re-shape SubscriptionStateResponse around current/tiers/
  usage; add ok/error/message/payload envelope + idempotency_key to the
  action responses; add SubscriptionCurrent / SubscriptionTier / preview
  payload types. All nullable + ignoreUnknownKeys (graceful degrade).
- BillingViewModel: detect ok:false results on every call and route to
  featureUnavailable / errorMessage instead of swallowing them.
- BillingScreen: read plan from subscription.current; render a clean
  'no active plan' / 'log in to Nous Portal' card on the logged-out path
  instead of a broken null plan card.
- strings: add billing_no_active_plan / billing_login_required (en + ko).

Verified: ./gradlew ktlintCheck + compileDebugKotlin both BUILD SUCCESSFUL.

* fix(billing): address independent review (agy) — error routing + icon

Triage of the independent AGY review on PR #629:

Fixed (real issues):
- BillingViewModel: usage.bars RPC failure (-32601) no longer marks the
  whole feature unavailable (was blocking the plan card despite the
  'non-fatal' comment); it is now correctly non-fatal.
- BillingViewModel: a failed primary subscription load now surfaces
  errorMessage even when usage bars loaded (was swallowed by
  'sub == null && bars == null').
- BillingScreen: render state.errorMessage as an error card inside
  BillingContent so failed mutating actions (upgrade/cancel) are visible
  instead of silently disappearing.
- BillingScreen: wrap the dead AccountBalanceWallet expression in a real
  Icon() composable in FeatureUnavailableState.

Declined (with rationale, recorded for maintainer):
- Tier-select / upgrade UI (review issue #4): explicitly out of scope per
  the PR body — 'Plan-selection UI ... is not built ... left for a
  follow-up once backend returns the available plans.'
- Rename snake_case Kotlin props to camelCase (review nit #2): incorrect —
  OkHttpProvider uses JsonNamingStrategy.SnakeCase, which maps snake_case
  wire keys onto snake_case properties; the props are correct. Fixed the
  misleading doc comment instead.

* fix(billing): match REAL logged-in wire shapes + kill null-result crash

The live gateway probe (logged-in Nous Portal session, 2026-07-19) showed
the earlier models assumed the WRONG shape:

- subscription.state.current is {tier_id, tier_name, monthly_credits,
  credits_remaining, cycle_ends_at, pending_downgrade_*} — NOT the legacy
  subscription_type_id/status/renews_at fields. Screen rendered 'Free / no
  plan' for a real logged-in user.
- usage.bars is {available, status, plan_name, plan_bar{remaining_display,
  total_display, pct_used, fill_fraction}, topup_bar} — NOT a List<UsageBar>.
- Mutating RPCs return error:'insufficient_scope' when the Portal token
  lacks billing:manage scope (ok:false result, not an RPC error).

Crash fix (root cause of 'Billing RPC returned no result payload'):
- BillingRepository.decode() now returns null on a missing/null result
  instead of requireNotNull() -> IllegalStateException. All repo methods
  return nullable; ViewModel handles null as a non-fatal 'No billing data
  returned' instead of crashing.
- BillingScreen.PlanCard / UsageBarRow rewritten to the real fields; the
  resume/cancel branch now gates on can_change_plan (no status field exists
  on current anymore).
- UsageBarsResponse + UsageBar reshaped to plan_bar/topup_bar; added
  billing_credits_remaining / billing_plan / billing_topup strings (en+ko).

Verified: ktlint clean + compileDebugKotlin BUILD SUCCESSFUL. Decode sim
against the captured live responses confirms PlanCard/UsageBarRow/error
paths all resolve with no null-crash.

* fix(billing): decode WS result that arrives as toAny() Map, not JsonElement

Root cause of 'No billing data returned' on a logged-in server: the backend
returns a valid subscription.state result, but HermesWsClient.request()
yields result already converted via JsonElement.toAny() (a Map<String,Any?>),
not a raw JsonElement. BillingRepository.decode() only accepted JsonElement,
so  was null and the repo returned null -> ViewModel showed
the generic 'No billing data returned' message. The server DOES have billing
(logged_in:true, tier_name:Free, /usr/bin/bash.10 credits) — the app just couldn't parse it.

Fix: decode() now normalizes whatever arrives — a kotlinx JsonElement OR the
toAny() Map/List — back into a JsonElement via a recursive Any->JsonElement
walk, then deserializes. Both shapes decode correctly now.

This also means the previous 'Billing RPC returned no result payload' crash
and the generic 'no data' message were both symptoms of this same parse gap,
not a missing backend feature.

Verified: ktlint clean + compileDebugKotlin BUILD SUCCESSFUL.

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/NavigationKeys.kt

@@ -55,6 +55,8 @@ import kotlinx.serialization.Serializable
 
 @Serializable data object AnalyticsScreen : NavKey
 
+@Serializable data object BillingScreen : NavKey
+
 // ── Settings drill-down sub-pages ──────────────────────────────────────
 
 @Serializable data object SettingsConnection : NavKey