feat(chat): scroll-to-latest button + panic hold-progress ring (v1.10.0+14)

B6 scroll-to-latest: ScrollController on the timeline; a small
  arrow-down FAB fades in once scrolled >400px up and animates back
  to the newest message on tap (overlaid on the list, not the FAB slot)
B5 panic hold-ring: 700ms AnimationController drives a red progress
  ring around the panic icon; haptic + fire on completion, silent
  reset on early release; tap-hint and configured guard preserved

Panic ring verified in-browser (ring fills, alert fires to the room).
Scroll-to-latest verified with a deterministic widget test because the
CanvasKit surface rejects synthetic scroll input in headless Chromium.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/features/panic/presentation/panic_button.dart

@@ -1,9 +1,11 @@
-// Version: 1.0.0 | Created: 2026-07-04
-// Panic trigger button for the rooms AppBar. Push-to-talk style:
-// HOLD to fire the alert; a plain tap only shows the hint. Only rendered
-// when a panic configuration exists in account data.
+// Version: 1.1.0 | Created: 2026-07-04 | Updated: 2026-07-04
+// Panic trigger button for the rooms AppBar. Push-to-talk style: HOLD for
+// 700ms to fire the alert, with a filling ring showing progress so a brush
+// cannot fire it and the user sees exactly when it sends. A plain tap only
+// shows the hint. Only rendered when a panic configuration exists.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../data/panic_repository.dart';
@@ -15,9 +17,47 @@ class PanicButton extends ConsumerStatefulWidget {
   ConsumerState<PanicButton> createState() => _PanicButtonState();
 }
 
-class _PanicButtonState extends ConsumerState<PanicButton> {
+class _PanicButtonState extends ConsumerState<PanicButton>
+    with SingleTickerProviderStateMixin {
+  static const _holdDuration = Duration(milliseconds: 700);
+
+  late final AnimationController _holdController;
   bool _sending = false;
 
+  @override
+  void initState() {
+    super.initState();
+    _holdController = AnimationController(vsync: this, duration: _holdDuration)
+      ..addStatusListener(_onHoldStatus);
+  }
+
+  @override
+  void dispose() {
+    _holdController.dispose();
+    super.dispose();
+  }
+
+  void _onHoldStatus(AnimationStatus status) {
+    // The ring filled all the way: the hold completed, fire the alert.
+    if (status == AnimationStatus.completed) {
+      HapticFeedback.heavyImpact();
+      _trigger();
+      _holdController.reset();
+    }
+  }
+
+  void _onHoldStart() {
+    if (_sending) return;
+    _holdController.forward(from: 0);
+  }
+
+  void _onHoldCancelled() {
+    // Released before the ring filled: cancel silently.
+    if (_holdController.status == AnimationStatus.forward) {
+      _holdController.reset();
+    }
+  }
+
   Future<void> _trigger() async {
     if (_sending) return;
     setState(() => _sending = true);
@@ -32,9 +72,15 @@ class _PanicButtonState extends ConsumerState<PanicButton> {
     messenger.showSnackBar(
       SnackBar(
         backgroundColor: error == null ? Colors.green.shade800 : null,
-        content: Text(
-          error ?? 'Emergency alert sent with your location.',
-        ),
+        content: Text(error ?? 'Emergency alert sent with your location.'),
+      ),
+    );
+  }
+
+  void _showHint() {
+    ScaffoldMessenger.of(context).showSnackBar(
+      const SnackBar(
+        content: Text('Hold the button to send the emergency alert.'),
       ),
     );
   }
@@ -47,23 +93,54 @@ class _PanicButtonState extends ConsumerState<PanicButton> {
     if (!configured) return const SizedBox.shrink();
 
     return GestureDetector(
-      onLongPress: _trigger,
-      child: IconButton(
-        tooltip: 'Hold to send emergency alert',
-        icon: _sending
-            ? const SizedBox(
-                width: 20,
-                height: 20,
-                child: CircularProgressIndicator(strokeWidth: 2),
-              )
-            : const Icon(Icons.emergency_share, color: Colors.red),
-        onPressed: () {
-          ScaffoldMessenger.of(context).showSnackBar(
-            const SnackBar(
-              content: Text('Hold the button to send the emergency alert.'),
+      onTap: _showHint,
+      onLongPressDown: (_) => _onHoldStart(),
+      onLongPressUp: _onHoldCancelled,
+      onLongPressCancel: _onHoldCancelled,
+      child: Tooltip(
+        message: 'Hold to send emergency alert',
+        child: SizedBox(
+          width: 48,
+          height: 48,
+          child: Center(
+            child: SizedBox(
+              width: 40,
+              height: 40,
+              child: Stack(
+                alignment: Alignment.center,
+                children: [
+                  // Progress ring, only painted while the hold is in progress.
+                  AnimatedBuilder(
+                    animation: _holdController,
+                    builder: (context, _) {
+                      if (_holdController.value == 0) {
+                        return const SizedBox.shrink();
+                      }
+                      return SizedBox(
+                        width: 36,
+                        height: 36,
+                        child: CircularProgressIndicator(
+                          value: _holdController.value,
+                          strokeWidth: 3,
+                          color: Colors.red,
+                          backgroundColor: Colors.red.withAlpha(40),
+                        ),
+                      );
+                    },
+                  ),
+                  if (_sending)
+                    const SizedBox(
+                      width: 20,
+                      height: 20,
+                      child: CircularProgressIndicator(strokeWidth: 2),
+                    )
+                  else
+                    const Icon(Icons.emergency_share, color: Colors.red),
+                ],
+              ),
             ),
-          );
-        },
+          ),
+        ),
       ),
     );
   }