feat: custom native file picker showing hidden files/folders

Replaced file_picker's Load/Browse with a custom NativePickerRegistrar
Swift plugin that opens NSOpenPanel with showsHiddenFiles = true.
The file_picker package hardcodes this to false, making hidden folders
like ~/.claude invisible in its dialogs.

Changes:
- New NativePickerRegistrar.swift: custom NSOpenPanel with hidden files
- New NativePicker Dart service using method channel
- Browse: only shows folders (canChooseFiles=false), hidden visible
- Load: only shows .jsonl files, hidden folders visible
- Registered via AppDelegate.applicationDidFinishLaunching
- Removed file_picker dependency from home_screen imports
- Fixed all info-level lint issues (super params, null-aware, doc comment)
- Signed, notarized, stapled DMG

lib/models/content_block.dart

@@ -22,8 +22,8 @@ class ContentBlock {
 class TextBlock extends ContentBlock {
   final String text;
 
-  TextBlock({required this.text, required Map<String, dynamic> raw})
-      : super(type: 'text', raw: raw);
+  TextBlock({required this.text, required super.raw})
+      : super(type: 'text');
 
   factory TextBlock.fromJson(Map<String, dynamic> json) {
     return TextBlock(
@@ -40,8 +40,8 @@ class ThinkingBlock extends ContentBlock {
   ThinkingBlock({
     required this.thinking,
     this.signature,
-    required Map<String, dynamic> raw,
-  }) : super(type: 'thinking', raw: raw);
+    required super.raw,
+  }) : super(type: 'thinking');
 
   factory ThinkingBlock.fromJson(Map<String, dynamic> json) {
     return ThinkingBlock(
@@ -63,8 +63,8 @@ class ToolUseBlock extends ContentBlock {
     required this.name,
     required this.input,
     this.linkedResult,
-    required Map<String, dynamic> raw,
-  }) : super(type: 'tool_use', raw: raw);
+    required super.raw,
+  }) : super(type: 'tool_use');
 
   factory ToolUseBlock.fromJson(Map<String, dynamic> json) {
     return ToolUseBlock(