Add Draw mode for Apple Pencil annotation on scripture panel

Adds a third layout option alongside Stacked and Split. In Draw mode a
canvas overlay covers the scripture panel; the Pointer Events API routes
Apple Pencil input to perfect-freehand strokes while letting finger
touches fall through for normal scrolling. A floating toolbar provides
pen, highlighter, and eraser tools, six color swatches, S/M/L sizes,
undo, clear, and a Done button to return to Stacked view.

Strokes are stored as inkStrokes[] on each chunk and rendered as a
read-only transparent canvas overlay (InkLayer) in Stacked/Split modes.
migrateChunk is updated so all existing chunks get inkStrokes: [] on
first load.

Also adds a simple in-memory rate limiter on auth endpoints (10 attempts
per 15 min per IP).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -26,6 +26,7 @@
     "express-session": "^1.19.0",
     "mammoth": "^1.12.0",
     "otplib": "^12.0.1",
+    "perfect-freehand": "^1.2.3",
     "qrcode": "^1.5.4",
     "react": "^19.0.0",
     "react-dom": "^19.0.0"