Phase 3 implementation. Coordinate system implemented. Initial text box tool implemented.

frontend/src/lib/coords.ts

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+export interface PdfPoint { x: number; y: number }
+export interface ScreenPoint { x: number; y: number }
+
+export interface PdfRect { x: number; y: number; width: number; height: number }
+export interface ScreenRect { x: number; y: number; width: number; height: number }
+
+export interface ViewportParams {
+  scale: number;          // CSS Zoom level (1.0 = 100%)
+  rotation: number;       // 0, 90, 180, 270 (clockwise)
+  canonicalWidth: number; // Unrotated CropBox width in PDF points
+  canonicalHeight: number;// Unrotated CropBox height in PDF points
+  dpr?: number;           // Device Pixel Ratio (defaults to 1)
+}
+
+function getEffectiveScale(vp: ViewportParams): number {
+  return vp.scale * (vp.dpr || 1);
+}
+
+export function pdfToScreen(p: PdfPoint, vp: ViewportParams): ScreenPoint {
+  const s = getEffectiveScale(vp);
+  const { rotation, canonicalWidth: W, canonicalHeight: H } = vp;
+  const { x, y } = p;
+  
+  const rot = ((rotation % 360) + 360) % 360;
+
+  switch (rot) {
+    case 0:   return { x: x * s, y: y * s };
+    case 90:  return { x: (H - y) * s, y: x * s };
+    case 180: return { x: (W - x) * s, y: (H - y) * s };
+    case 270: return { x: y * s, y: (W - x) * s };
+    default:  throw new Error(`Invalid rotation: ${rotation}`);
+  }
+}
+
+export function screenToPdf(p: ScreenPoint, vp: ViewportParams): PdfPoint {
+  const s = getEffectiveScale(vp);
+  const { rotation, canonicalWidth: W, canonicalHeight: H } = vp;
+  const sx = p.x / s;
+  const sy = p.y / s;
+
+  const rot = ((rotation % 360) + 360) % 360;
+
+  switch (rot) {
+    case 0:   return { x: sx, y: sy };
+    case 90:  return { x: sy, y: H - sx };
+    case 180: return { x: W - sx, y: H - sy };
+    case 270: return { x: W - sy, y: sx };
+    default:  throw new Error(`Invalid rotation: ${rotation}`);
+  }
+}
+
+export function pdfRectToScreen(r: PdfRect, vp: ViewportParams): ScreenRect {
+  const p1 = pdfToScreen({ x: r.x, y: r.y }, vp);
+  const p2 = pdfToScreen({ x: r.x + r.width, y: r.y + r.height }, vp);
+
+  return {
+    x: Math.min(p1.x, p2.x),
+    y: Math.min(p1.y, p2.y),
+    width: Math.abs(p2.x - p1.x),
+    height: Math.abs(p2.y - p1.y)
+  };
+}
+
+export function screenRectToPdf(r: ScreenRect, vp: ViewportParams): PdfRect {
+  const p1 = screenToPdf({ x: r.x, y: r.y }, vp);
+  const p2 = screenToPdf({ x: r.x + r.width, y: r.y + r.height }, vp);
+
+  return {
+    x: Math.min(p1.x, p2.x),
+    y: Math.min(p1.y, p2.y),
+    width: Math.abs(p2.x - p1.x),
+    height: Math.abs(p2.y - p1.y)
+  };
+}