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/app/api/labels/[id]/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server'
 import prisma from '@/lib/prisma'
+import { revalidatePath } from 'next/cache'
 
 // GET /api/labels/[id] - Get a specific label
 export async function GET(
@@ -42,14 +43,68 @@ export async function PUT(
     const body = await request.json()
     const { name, color } = body
 
+    // Get the current label first
+    const currentLabel = await prisma.label.findUnique({
+      where: { id }
+    })
+
+    if (!currentLabel) {
+      return NextResponse.json(
+        { success: false, error: 'Label not found' },
+        { status: 404 }
+      )
+    }
+
+    const newName = name ? name.trim() : currentLabel.name
+
+    // If renaming, update all notes that use this label
+    if (name && name.trim() !== currentLabel.name) {
+      // Get all notes that use this label
+      const allNotes = await prisma.note.findMany({
+        where: {
+          userId: currentLabel.userId,
+          labels: { not: null }
+        },
+        select: { id: true, labels: true }
+      })
+
+      // Update the label name in all notes that use it
+      for (const note of allNotes) {
+        if (note.labels) {
+          try {
+            const noteLabels: string[] = JSON.parse(note.labels)
+            const updatedLabels = noteLabels.map(l =>
+              l.toLowerCase() === currentLabel.name.toLowerCase() ? newName : l
+            )
+
+            // Update the note if labels changed
+            if (JSON.stringify(updatedLabels) !== JSON.stringify(noteLabels)) {
+              await prisma.note.update({
+                where: { id: note.id },
+                data: {
+                  labels: JSON.stringify(updatedLabels)
+                }
+              })
+            }
+          } catch (e) {
+            console.error(`Failed to parse labels for note ${note.id}:`, e)
+          }
+        }
+      }
+    }
+
+    // Now update the label record
     const label = await prisma.label.update({
       where: { id },
       data: {
-        ...(name && { name: name.trim() }),
+        ...(name && { name: newName }),
         ...(color && { color })
       }
     })
 
+    // Revalidate to refresh UI
+    revalidatePath('/')
+
     return NextResponse.json({
       success: true,
       data: label
@@ -70,13 +125,63 @@ export async function DELETE(
 ) {
   try {
     const { id } = await params
+
+    // First, get the label to know its name and userId
+    const label = await prisma.label.findUnique({
+      where: { id }
+    })
+
+    if (!label) {
+      return NextResponse.json(
+        { success: false, error: 'Label not found' },
+        { status: 404 }
+      )
+    }
+
+    // Get all notes that use this label
+    const allNotes = await prisma.note.findMany({
+      where: {
+        userId: label.userId,
+        labels: { not: null }
+      },
+      select: { id: true, labels: true }
+    })
+
+    // Remove the label from all notes that use it
+    for (const note of allNotes) {
+      if (note.labels) {
+        try {
+          const noteLabels: string[] = JSON.parse(note.labels)
+          const filteredLabels = noteLabels.filter(
+            l => l.toLowerCase() !== label.name.toLowerCase()
+          )
+
+          // Update the note if labels changed
+          if (filteredLabels.length !== noteLabels.length) {
+            await prisma.note.update({
+              where: { id: note.id },
+              data: {
+                labels: filteredLabels.length > 0 ? JSON.stringify(filteredLabels) : null
+              }
+            })
+          }
+        } catch (e) {
+          console.error(`Failed to parse labels for note ${note.id}:`, e)
+        }
+      }
+    }
+
+    // Now delete the label record
     await prisma.label.delete({
       where: { id }
     })
 
+    // Revalidate to refresh UI
+    revalidatePath('/')
+
     return NextResponse.json({
       success: true,
-      message: 'Label deleted successfully'
+      message: `Label "${label.name}" deleted and removed from ${allNotes.length} notes`
     })
   } catch (error) {
     console.error('DELETE /api/labels/[id] error:', error)