New mode: Point-to-point distance with right-angle triangle

[pgivel]

Summary

Add a two-click measurement mode: place point A, place point B, and display the direct distance with an optional right-angle triangle showing Δx and Δy decomposition.

Use case

Measuring distances between elements that are not aligned or not adjacent — e.g. "how far is this icon from that label?" Handles interrupted or diagonal measurements that the crosshair mode cannot.

Interaction

1. User selects point-to-point mode (keyboard shortcut, e.g. 6).
2. Click to place point A (auto-snapped via existing snapPointToNearestEdge).
3. While moving to point B, a live preview line from A to cursor is drawn, with a live distance readout.
4. Click to place point B (also auto-snapped).
5. The measurement annotation is displayed.

Display

• A line segment from A to B.
• Distance label on the line: e.g. 143.4 px.
• Right-angle triangle (if the line is not near-axis-aligned):
  • Drawn only when min(|Δx|, |Δy|) > threshold (e.g. 8 px), otherwise it's nearly a straight horizontal/vertical line and the triangle adds clutter.
  • Shows the horizontal leg (Δx), vertical leg (Δy), and the hypotenuse (total distance).
  • Each leg labeled with its length.
  • Triangle drawn with dashed or lighter lines to distinguish from the main measurement line.

Implementation notes

• Two-click state machine in QML: idle → waitingSecondPoint → placed.
• Reuse snapPointToNearestEdge for both clicks.
• Distance calculation: sqrt(Δx² + Δy²) in logical pixels. Use backend DPR for coordinate mapping.
• Add a new @pyqtSlot or compute purely on the QML/JS side since it's just Pythagoras.
• Visual components: Canvas for the line + triangle, MeasurementLabel for the text.

Quick mode vs Session mode

• Quick mode: After placing both points, the measurement text is copied to clipboard and the app quits. Format: A(x1, y1) → B(x2, y2) — 143.4 px (Δx=140, Δy=25).
• Session mode (Session mode: persistent annotations, measurement export & composite screenshot #6): Annotation persists; user can place additional measurements.