Adapt layout for wide screens and tablets

- Switch navigation breakpoint from orientation to width (≥600dp):
  NavigationRail shown on tablets in portrait and phones in landscape
- Chat bubble max-width capped at 480dp (prevents 640dp+ bubbles on tablets)
- Chat input maxLines reduced to 2 when width ≥600dp
- Task edit form centered and constrained to 600dp max-width on tablets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/tasks/task_edit_screen.dart

@@ -147,10 +147,14 @@ class _TaskEditScreenState extends ConsumerState<TaskEditScreen> {
           ),
           body: snapshot.connectionState == ConnectionState.waiting
               ? const Center(child: CircularProgressIndicator())
-              : Form(
-                  key: _formKey,
-                  child: ListView(
-                    padding: const EdgeInsets.all(16),
+              : Align(
+                  alignment: Alignment.topCenter,
+                  child: ConstrainedBox(
+                    constraints: const BoxConstraints(maxWidth: 600),
+                    child: Form(
+                      key: _formKey,
+                      child: ListView(
+                        padding: const EdgeInsets.all(16),
                     children: [
                       TextFormField(
                         controller: _titleController,
@@ -215,6 +219,8 @@ class _TaskEditScreenState extends ConsumerState<TaskEditScreen> {
                     ],
                   ),
                 ),
+                  ),
+                ),
         );
       },
     );