feat(studio): clear-audit-log button on Audit page (v0.15.0)

Operators can now clear dev-noise from the audit log without
dropping to the CLI. Top-bar broom icon next to the filter
chips opens a two-field dialog (reviewer pre-filled from $USER,
reason required). On submit the hub call returns the purged
count + active channel; both are shown back in a snackbar.

Compliance-locked channels (beta/production) are refused
server-side; the dialog surfaces the rejection reason via a
friendly error message instead of the raw gRPC status string.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/pages/audit.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:io';
 
 import 'package:flutter/material.dart';
 
@@ -38,6 +39,42 @@ class _AuditPageState extends State<AuditPage> {
     super.dispose();
   }
 
+  Future<void> _onClearPressed() async {
+    final outcome = await _ClearAuditDialog.show(context);
+    if (outcome == null || !mounted) return;
+    try {
+      final r = await HubService.instance.clearEventLog(
+        reviewer: outcome.reviewer,
+        reason: outcome.reason,
+      );
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text(
+            'Cleared ${r.purged} events on channel "${r.channel}". '
+            'Chain reseeded with marker.',
+          ),
+        ),
+      );
+      _refresh();
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('Clear failed: ${_friendly(e)}')),
+      );
+    }
+  }
+
+  String _friendly(Object e) {
+    final s = e.toString();
+    // The hub returns FailedPrecondition for compliance-locked
+    // channels; tonic stringifies that as "failed_precondition:
+    // …". Strip the noise so the operator sees the real reason.
+    final marker = 'failed_precondition: ';
+    final idx = s.toLowerCase().indexOf(marker);
+    return idx >= 0 ? s.substring(idx + marker.length) : s;
+  }
+
   Future<void> _refresh() async {
     try {
       final events = await HubService.instance.recentEvents(limit: 100);
@@ -75,6 +112,12 @@ class _AuditPageState extends State<AuditPage> {
               onChanged: (v) => setState(() => _typeFilter = v),
             ),
           ),
+          IconButton(
+            icon: const Icon(Icons.delete_sweep_outlined, size: 18),
+            tooltip: 'Clear log (local/dev channels only)',
+            onPressed: _onClearPressed,
+          ),
+          const SizedBox(width: FaiSpace.sm),
         ],
       ),
       body: Column(
@@ -687,3 +730,126 @@ class _ExplanationPanel extends StatelessWidget {
     );
   }
 }
+
+/// Outcome of the clear-audit confirmation dialog.
+class _ClearOutcome {
+  final String reviewer;
+  final String reason;
+  const _ClearOutcome({required this.reviewer, required this.reason});
+}
+
+/// Two-field confirmation dialog for "clear the audit log".
+/// Reviewer defaults to the OS user (closest stable identity
+/// without an auth backend); reason has no default so the
+/// operator has to type *something* — the chain.reset marker
+/// must carry context.
+class _ClearAuditDialog extends StatefulWidget {
+  const _ClearAuditDialog();
+
+  static Future<_ClearOutcome?> show(BuildContext context) {
+    return showDialog<_ClearOutcome>(
+      context: context,
+      builder: (_) => const _ClearAuditDialog(),
+    );
+  }
+
+  @override
+  State<_ClearAuditDialog> createState() => _ClearAuditDialogState();
+}
+
+class _ClearAuditDialogState extends State<_ClearAuditDialog> {
+  late final TextEditingController _reviewer;
+  late final TextEditingController _reason;
+
+  @override
+  void initState() {
+    super.initState();
+    final user = Platform.environment['USER'] ??
+        Platform.environment['USERNAME'] ??
+        'operator';
+    _reviewer = TextEditingController(text: '$user@studio');
+    _reason = TextEditingController();
+  }
+
+  @override
+  void dispose() {
+    _reviewer.dispose();
+    _reason.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return AlertDialog(
+      title: const Text('Clear audit log?'),
+      content: ConstrainedBox(
+        constraints: const BoxConstraints(maxWidth: 460),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text(
+              'Wipes every audit event on the active channel and seeds a '
+              'fresh chain.reset marker carrying reviewer + reason. '
+              'Refused on beta / production. Irreversible.',
+              style: theme.textTheme.bodySmall?.copyWith(
+                color: theme.colorScheme.onSurfaceVariant,
+              ),
+            ),
+            const SizedBox(height: FaiSpace.lg),
+            TextField(
+              controller: _reviewer,
+              decoration: const InputDecoration(
+                labelText: 'Reviewer',
+                border: OutlineInputBorder(),
+                isDense: true,
+              ),
+            ),
+            const SizedBox(height: FaiSpace.md),
+            ValueListenableBuilder<TextEditingValue>(
+              valueListenable: _reason,
+              builder: (_, _, _) => TextField(
+                controller: _reason,
+                autofocus: true,
+                decoration: InputDecoration(
+                  labelText: 'Reason (recorded in chain.reset marker)',
+                  helperText: _reason.text.trim().isEmpty
+                      ? 'Required — auditors will read this.'
+                      : null,
+                  border: const OutlineInputBorder(),
+                  isDense: true,
+                ),
+              ),
+            ),
+          ],
+        ),
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(context, null),
+          child: const Text('Cancel'),
+        ),
+        ValueListenableBuilder<TextEditingValue>(
+          valueListenable: _reason,
+          builder: (_, _, _) => FilledButton(
+            onPressed: _reason.text.trim().isEmpty
+                ? null
+                : () => Navigator.pop(
+                      context,
+                      _ClearOutcome(
+                        reviewer: _reviewer.text.trim(),
+                        reason: _reason.text.trim(),
+                      ),
+                    ),
+            style: FilledButton.styleFrom(
+              backgroundColor: Theme.of(context).colorScheme.error,
+              foregroundColor: Theme.of(context).colorScheme.onError,
+            ),
+            child: const Text('Clear log'),
+          ),
+        ),
+      ],
+    );
+  }
+}