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/prisma/schema.prisma

@@ -37,6 +37,7 @@ model User {
   sessions           Session[]
   aiSettings         UserAISettings?
   workflows          Workflow[]
+  notifications      Notification[]
 }
 
 model Account {
@@ -278,6 +279,7 @@ model UserAISettings {
   noteHistory          Boolean @default(false)
   noteHistoryMode      String  @default("manual")
   languageDetection    Boolean @default(true)
+  fontFamily           String  @default("inter")
   user                 User    @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([memoryEcho])
@@ -409,3 +411,19 @@ model WorkflowRun {
   @@index([workflowId])
   @@index([status])
 }
+
+model Notification {
+  id         String   @id @default(cuid())
+  userId     String
+  type       String   // "agent_success", "agent_failure", "share", "reminder", "system"
+  title      String
+  message    String?
+  read       Boolean  @default(false)
+  actionUrl  String?  // e.g. "/agents" or "/notes/xxx"
+  relatedId  String?  // e.g. agentId or noteId
+  createdAt  DateTime @default(now())
+  user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId, read])
+  @@index([userId, createdAt])
+}