feat(trust): full Duniter web-of-trust membership (params + referents)

Slice 4 of Block 2. The pure rule was already parameterised; this adds the
policy, cold-start and UI around it.

- commons_core: WotParams (sigQty/stepMax/sigValidity, Duniter defaults) +
  WebOfTrust.membersWith; npubToHex helper (NIP-19 decode) for adding roots.
- WotSettings (keystore): the active parameters, configurable — a young
  network loosens them and tightens as it grows, as Ğ1 did. Defaults Duniter.
- TrustReferents: the bootstrap 'seeds' membership is measured from — a
  bundled asset (empty until real founders are curated, no invented keys)
  unioned with referents the user adds by npub/QR. Honest cold-start.
- TrustCubit: computes the full membership verdict against referents+params
  alongside the personal circle, and exposes a TrustTier (networkMember >
  inYourCircle > vouched > unknown). Certifications issued with the active
  validity (they expire and renew, Duniter rule).
- UI: chat trust badge by tier; a 'Network of trust' screen (manage roots +
  advanced params) reached from the profile. i18n en/es/pt/ast.

Tests: WotParams/membersWith, npubToHex, TrustReferents, WotSettings, and
TrustCubit tiers/membership. Resolves the WoT-parameters decision
(open-decisions §B). Trust net stays empty/undetermined until seeded — by
design; users bootstrap their own roots.

apps/app_seeds/lib/services/wot_settings.dart

@@ -0,0 +1,45 @@
+import 'package:commons_core/commons_core.dart';
+
+import '../security/secret_store.dart';
+
+/// Persists the web-of-trust parameters (Duniter sigQty / stepMax / validity),
+/// so a young network can loosen them and tighten as it grows — exactly how Ğ1
+/// itself evolved. Defaults to [WotParams.duniter]. Keystore-backed.
+class WotSettings {
+  WotSettings(this._store);
+
+  final SecretStore _store;
+
+  static const _sigQtyKey = 'tane.social.wot.sigQty';
+  static const _stepMaxKey = 'tane.social.wot.stepMax';
+  static const _validityDaysKey = 'tane.social.wot.validityDays';
+
+  /// The active parameters (any unset field falls back to the Duniter default).
+  Future<WotParams> params() async {
+    const base = WotParams.duniter;
+    final sigQty = await _readInt(_sigQtyKey) ?? base.sigQty;
+    final stepMax = await _readInt(_stepMaxKey) ?? base.stepMax;
+    final validityDays =
+        await _readInt(_validityDaysKey) ?? base.sigValidity.inDays;
+    return WotParams(
+      sigQty: sigQty,
+      stepMax: stepMax,
+      sigValidity: Duration(days: validityDays),
+    );
+  }
+
+  Future<void> save(WotParams params) async {
+    await _store.write(_sigQtyKey, '${params.sigQty}');
+    await _store.write(_stepMaxKey, '${params.stepMax}');
+    await _store.write(_validityDaysKey, '${params.sigValidity.inDays}');
+  }
+
+  /// Restores the Ğ1 reference values.
+  Future<void> reset() => save(WotParams.duniter);
+
+  Future<int?> _readInt(String key) async {
+    final raw = await _store.read(key);
+    if (raw == null || raw.isEmpty) return null;
+    return int.tryParse(raw);
+  }
+}