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/migrations/20260501000000_add_notification_model/migration.sql

@@ -0,0 +1,23 @@
+-- CreateTable
+CREATE TABLE "Notification" (
+    "id" TEXT NOT NULL,
+    "userId" TEXT NOT NULL,
+    "type" TEXT NOT NULL,
+    "title" TEXT NOT NULL,
+    "message" TEXT,
+    "read" BOOLEAN NOT NULL DEFAULT false,
+    "actionUrl" TEXT,
+    "relatedId" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "Notification_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "Notification_userId_read_idx" ON "Notification"("userId", "read");
+
+-- CreateIndex
+CREATE INDEX "Notification_userId_createdAt_idx" ON "Notification"("userId", "createdAt");
+
+-- AddForeignKey
+ALTER TABLE "Notification" ADD CONSTRAINT "Notification_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;