New mode: Color picker

[pgivel]

Summary

Add a color picker mode that samples the pixel color under the cursor and drops an annotation with hex and RGB values.

Use case

Quick color checks during UI validation: "Is this the right brand color?", "Do these two elements use the same color?". Keeps the tool in-scope as a screen measurement utility — color is just another property to measure.

Interaction

1. User selects color picker mode (keyboard shortcut, e.g. 5).
2. Cursor shows a small live preview swatch of the color under the pointer.
3. Click places a color annotation at that point.
4. The annotation displays:
   • A color swatch (small filled rectangle)
   • Hex value: #FF5722
   • RGB value: rgb(255, 87, 34)

Implementation notes

• The screenshot QImage is already available in RulerBackend. Use QImage.pixelColor(x, y) to sample.
• Expose a @pyqtSlot like sampleColorAtPoint(x, y) returning hex/RGB strings (or a structured object).
• QML: small swatch rectangle + text label, positioned near cursor. Reuse MeasurementLabel layout patterns.
• In session mode (Session mode: persistent annotations, measurement export & composite screenshot #6), clicking places a persistent color annotation.
• In quick mode, clicking copies the color string to clipboard and quits.

Future extensions (out of scope for this issue)

• Comparing two picked colors: show WCAG contrast ratio, ΔE color distance.
• Average color over a small region (e.g. 3×3 or 5×5 around cursor) to reduce single-pixel noise.
• Linking two color annotations to highlight match/mismatch.