feat: add flutter linter rules for widget best practices

Add use_super_parameters, use_key_in_widget_constructors, and
avoid_positional_boolean_parameters to enforce modern Dart idioms and
prevent widget state-loss bugs.

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

LATER.md

@@ -37,12 +37,6 @@ Why is that still needed? We have Stalwart.
 
 ---
 
-du -a | sort -rn | head
-
-Why is the directory so big? 4 GByte?
-
----
-
 After Try Connection, show some matching icon next to the text.
 
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@@ -61,14 +55,6 @@ Sieve: JMAP, easy. Per IMAP...
 
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-Flutter best practices?
-
----
-
-Flutter/Dart Linters?
-
----
-
 scripts/check_coverage.dart
 reduce files in _excluded.
 

analysis_options.yaml

@@ -52,6 +52,13 @@ linter:
     - discarded_futures
     - unnecessary_await_in_return
 
+    # Widget best practices
+    - use_super_parameters
+    - use_key_in_widget_constructors
+
+    # API clarity
+    - avoid_positional_boolean_parameters
+
     # Imports and style
     - directives_ordering
     - curly_braces_in_flow_control_structures