feat: 8 AI providers, rich text editor, agent notifications, UI contrast & font settings

- Add DeepSeek, OpenRouter, Mistral, Z.AI, LM Studio as AI providers
  with editable model names via Combobox in admin settings
- Fix OpenRouter broken by normalizeProvider bug in config.ts
- Convert agent-created notes from Markdown to HTML (TipTap rich text)
- Add Notification model + in-app notifications for agent results
- Agent notification click opens the created note directly
- Add note count display on notebook and inbox headers
- Fix checklist toggle in card view (persist state via localCheckItems)
- Add checklist creation option in tabs/list view (dropdown on + button)
- Fix image description ENOENT error with HTTP fallback
- Improve UI contrast across all themes (input, border, checkbox visibility)
- Add font family setting (Inter vs System Default) in Appearance settings
- Fix CSS font-sans variable conflict (removed dead Geist references)
- Update README with new features and 8 providers

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

memento-note/locales/ja.json

@@ -837,6 +837,11 @@
       "providerOllamaOption": "🦙 Ollama (Local & Free)",
       "providerOpenAIOption": "🤖 OpenAI (GPT-5, GPT-4)",
       "providerCustomOption": "🔧 Custom OpenAI-Compatible",
+      "providerDeepSeekOption": "🔍 DeepSeek",
+      "providerOpenRouterOption": "🌐 OpenRouter",
+      "providerMistralOption": "🌀 Mistral AI",
+      "providerZAIOption": "✨ Z.AI",
+      "providerLMStudioOption": "🖥️ LM Studio (ローカル)",
       "bestValue": "最もコスパが良い",
       "bestQuality": "最高品質",
       "saved": "(保存済み)",
@@ -1175,7 +1180,11 @@
     "notesViewLabel": "ノートのレイアウト",
     "notesViewTabs": "タブ（OneNote風）",
     "notesViewMasonry": "カード（グリッド）",
-    "selectTheme": "Select theme"
+    "selectTheme": "Select theme",
+    "fontFamilyLabel": "フォントファミリー",
+    "fontFamilyDescription": "アプリ全体で使用するフォントを選択してください",
+    "selectFontFamily": "Inter は読みやすさに最適化されています。システムはOSのネイティブフォントを使用します",
+    "fontSystem": "システムデフォルト"
   },
   "generalSettings": {
     "title": "一般設定",