Phase 4 Stage 2: onboarding redesign (client_app + mitra_app)

Verif Choice Sheet on display_name_screen drives the user into either
the verified or anonymous onboarding sub-flow. ESP screen (12 chips,
multi-select, info-only) + USP screen are shared between both branches;
selections persist through to chat_sessions.topics on session start.

OTP-blocked popup (HaloPopup) listens for the four real OTP-rate-limit
error codes (OTP_RATE_LIMIT_PHONE, OTP_RATE_LIMIT_IP, OTP_COOLDOWN,
OTP_ATTEMPTS_EXCEEDED) and drops the user onto the anonymous path with
ESP/USP state preserved.

Auth-providers gating replaces the --dart-define=ENABLE_SOCIAL_AUTH
build flag with server-driven discovery. authProvidersProvider preloads
GET /api/shared/auth-providers at cold start; welcome/register/
force-register screens render Google/Apple buttons only when the
backend reports enabled:true. Falls back to phone-OTP-only when both
providers are off. social_auth_enabled.dart deleted; client_app/CLAUDE.md
updated to reflect the new gating contract.

Mitra app: chat screen renders an ESP chip strip above the first message
bubble when chat_sessions.topics is non-empty.

Backend session.service.js getSessionById SELECTs cs.topics so the mitra
side can read the customer's selected topics.

Maestro flows 02_onboarding_verified.yaml + 03_onboarding_anon.yaml.

Deviation from plan: plan referenced OTP error code 'otp_retry_exhausted';
real codes are OTP_RATE_LIMIT_*/OTP_COOLDOWN/OTP_ATTEMPTS_EXCEEDED -
popup listens for all four. Plan said 'has_paid_first_session'; live
endpoint returns 'has_consulted_before' - used the live field.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/core/availability/mitra_availability_notifier.g.dart

@@ -6,9 +6,9 @@ part of 'mitra_availability_notifier.dart';
 // RiverpodGenerator
 // **************************************************************************
 
-String _$mitraAvailabilityHash() => r'7429862ccffbae1fc7bbe7368359e1624fe28ec9';
+String _$mitraAvailabilityHash() => r'036de8fea66c6a0114abd4a422af12186c1447d7';
 
-/// Phase 3.7 §1: customer-home availability poll.
+/// Customer-home availability poll.
 ///
 /// Polls `GET /api/client/mitra-availability` every 5 seconds while the home
 /// screen is in the foreground. Polling is gated by the home screen calling
@@ -16,10 +16,10 @@ String _$mitraAvailabilityHash() => r'7429862ccffbae1fc7bbe7368359e1624fe28ec9';
 ///   - resumed   → setActive(true)
 ///   - paused/inactive → setActive(false)
 ///
-/// On any HTTP error we emit `false` (PRD §1.3: never display stale state).
+/// On any HTTP error we emit `false` (never display stale state).
 ///
-/// The endpoint also returns a `count`, but per PRD §1.3 the customer UI must
-/// only read the binary `available` field — the count is for CC/debug only.
+/// The endpoint also returns a `count`, but the customer UI must only read the
+/// binary `available` field — the count is for CC/debug only.
 ///
 /// Copied from [MitraAvailability].
 @ProviderFor(MitraAvailability)