Add toast notification system with sonner library

- Create Toaster component wrapping sonner at bottom-right position
- Add showToast utility with success/error/info methods
- Error toasts persist until dismissed, others auto-dismiss after 5s
- Migrate error handling to toasts across all pages:
  - Dashboard (override toggle errors)
  - Settings (save/load success/error)
  - Garmin settings (connection success/error)
  - Calendar (load errors)
  - Period History (load/delete errors)
- Add dark mode support for toast styling
- Add Toaster provider to root layout
- 27 new tests (23 toaster component + 4 integration)
- Total: 977 unit tests passing

P5.2 COMPLETE - All P0-P5 items now complete.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pnpm-lock.yaml

@@ -47,6 +47,9 @@ importers:
       resend:
         specifier: ^6.7.0
         version: 6.7.0
+      sonner:
+        specifier: ^2.0.7
+        version: 2.0.7(react-dom@19.2.3(react@19.2.3))(react@19.2.3)
       tailwind-merge:
         specifier: ^3.4.0
         version: 3.4.0
@@ -72,6 +75,9 @@ importers:
       '@testing-library/react':
         specifier: ^16.3.1
         version: 16.3.1(@testing-library/dom@10.4.1)(@types/react-dom@19.2.3(@types/react@19.2.7))(@types/react@19.2.7)(react-dom@19.2.3(react@19.2.3))(react@19.2.3)
+      '@testing-library/user-event':
+        specifier: ^14.6.1
+        version: 14.6.1(@testing-library/dom@10.4.1)
       '@types/node':
         specifier: ^20
         version: 20.19.27
@@ -1230,6 +1236,12 @@ packages:
       '@types/react-dom':
         optional: true
 
+  '@testing-library/user-event@14.6.1':
+    resolution: {integrity: sha512-vq7fv0rnt+QTXgPxr5Hjc210p6YKq2kmdziLgnsZGgLJ9e6VAShx1pACLuRjd/AS/sr7phAR58OIIpf0LlmQNw==}
+    engines: {node: '>=12', npm: '>=6'}
+    peerDependencies:
+      '@testing-library/dom': '>=7.21.4'
+
   '@types/aria-query@5.0.4':
     resolution: {integrity: sha512-rfT93uj5s0PRL7EzccGMs3brplhcrghnDoV26NqKhCAS1hVo+WdNsPvE/yb6ilfr5hi2MEk6d5EWJTKdxg8jVw==}
 
@@ -1911,6 +1923,12 @@ packages:
   sonic-boom@4.2.0:
     resolution: {integrity: sha512-INb7TM37/mAcsGmc9hyyI6+QR3rR1zVRu36B0NeGXKnOOLiZOfER5SA+N7X7k3yUYRzLWafduTDvJAfDswwEww==}
 
+  sonner@2.0.7:
+    resolution: {integrity: sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==}
+    peerDependencies:
+      react: ^18.0.0 || ^19.0.0 || ^19.0.0-rc
+      react-dom: ^18.0.0 || ^19.0.0 || ^19.0.0-rc
+
   source-map-js@1.2.1:
     resolution: {integrity: sha512-UXWMKhLOwVKb728IUtQPXxfYU+usdybtUrK/8uGE8CQMvrhOpwvzDBwj0QhSL7MQc7vIsISBG8VQ8+IDQxpfQA==}
     engines: {node: '>=0.10.0'}
@@ -2934,6 +2952,10 @@ snapshots:
       '@types/react': 19.2.7
       '@types/react-dom': 19.2.3(@types/react@19.2.7)
 
+  '@testing-library/user-event@14.6.1(@testing-library/dom@10.4.1)':
+    dependencies:
+      '@testing-library/dom': 10.4.1
+
   '@types/aria-query@5.0.4': {}
 
   '@types/babel__core@7.20.5':
@@ -3608,6 +3630,11 @@ snapshots:
     dependencies:
       atomic-sleep: 1.0.0
 
+  sonner@2.0.7(react-dom@19.2.3(react@19.2.3))(react@19.2.3):
+    dependencies:
+      react: 19.2.3
+      react-dom: 19.2.3(react@19.2.3)
+
   source-map-js@1.2.1: {}
 
   source-map-support@0.5.21: