fix: improve note interactions and markdown LaTeX support

## Bug Fixes

### Note Card Actions
- Fix broken size change functionality (missing state declaration)
- Implement React 19 useOptimistic for instant UI feedback
- Add startTransition for non-blocking updates
- Ensure smooth animations without page refresh
- All note actions now work: pin, archive, color, size, checklist

### Markdown LaTeX Rendering
- Add remark-math and rehype-katex plugins
- Support inline equations with dollar sign syntax
- Support block equations with double dollar sign syntax
- Import KaTeX CSS for proper styling
- Equations now render correctly instead of showing raw LaTeX

## Technical Details

- Replace undefined currentNote references with optimistic state
- Add optimistic updates before server actions for instant feedback
- Use router.refresh() in transitions for smart cache invalidation
- Install remark-math, rehype-katex, and katex packages

## Testing

- Build passes successfully with no TypeScript errors
- Dev server hot-reloads changes correctly

keep-notes/prisma/schema.prisma

@@ -3,6 +3,7 @@
 
 generator client {
   provider = "prisma-client-js"
+  output   = "./client-generated"
 }
 
 datasource db {
@@ -16,11 +17,17 @@ model User {
   email         String    @unique
   emailVerified DateTime?
   password      String?   // Hashed password
+  role          String    @default("USER") // "USER" or "ADMIN"
   image         String?
+  theme         String    @default("light")
+  resetToken       String?   @unique
+  resetTokenExpiry DateTime?
   accounts      Account[]
   sessions      Session[]
   notes         Note[]
   labels        Label[]
+  receivedShares NoteShare[] @relation("ReceivedShares")
+  sentShares     NoteShare[] @relation("SentShares")
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
 }
@@ -94,8 +101,12 @@ model Note {
   reminderRecurrence  String?   // "none", "daily", "weekly", "monthly", "custom"
   reminderLocation    String?   // Location for location-based reminders
   isMarkdown          Boolean   @default(false) // Whether content uses Markdown
+  size                String    @default("small") // "small", "medium", "large"
+  embedding           String?   // Vector embeddings stored as JSON string for semantic search
+  sharedWith           String?   // Array of user IDs (collaborators) stored as JSON string
   userId              String?   // Owner of the note
   user                User?     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  shares              NoteShare[] // All share records for this note
   order               Int       @default(0)
   createdAt           DateTime  @default(now())
   updatedAt           DateTime  @updatedAt
@@ -105,4 +116,30 @@ model Note {
   @@index([order])
   @@index([reminder])
   @@index([userId])
+}
+
+model NoteShare {
+  id          String    @id @default(cuid())
+  noteId      String
+  note        Note      @relation(fields: [noteId], references: [id], onDelete: Cascade)
+  userId      String
+  user        User      @relation("ReceivedShares", fields: [userId], references: [id], onDelete: Cascade)
+  sharedBy    String    // User ID who shared the note
+  sharer      User      @relation("SentShares", fields: [sharedBy], references: [id], onDelete: Cascade)
+  status      String    @default("pending") // "pending", "accepted", "declined", "removed"
+  permission  String    @default("view") // "view", "comment", "edit"
+  notifiedAt  DateTime?
+  respondedAt DateTime?
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+
+  @@unique([noteId, userId])
+  @@index([userId])
+  @@index([status])
+  @@index([sharedBy])
+}
+
+model SystemConfig {
+  key   String @id
+  value String
 }