[Feature] Add Draw Shape tool for precise geometric shapes

Is your feature request related to a problem?

Currently, darktable supports freehand drawing for masks and paths, but lacks tools to draw precise geometric shapes (rectangles, circles, lines, polygons, or dots). This makes it difficult to:

• Create clean, straight-edged masks
• Draw perfect circles/ellipses for selections
• Add straight lines for guides or annotations
• Place exact points (dots) for marking specific locations

Describe the solution you'd like

Add a "Draw Shape" tool in the darkroom module with options to draw:
✅ Shapes: Rectangle, Circle, Ellipse, Line, Polygon
✅ Points: Single-click dot/point placement (for marking)
✅ Precision: Snap-to-grid, straight lines, perfect curves
✅ Integration: Work with existing masking/retouching workflows

Example:
Like the "Draw → Draw Shape" feature in Photo Editor Pro - Polish (see reference image), but tailored for darktable’s non-destructive editing.

Additional Context

• Use Cases:

  • Selective editing with geometric masks
  • Annotations for client reviews
  • Guides for composition/alignment
  • Marking focus points or areas of interest

• Alternatives:
  Users currently rely on freehand drawing (unprofessional/uneven) or external tools (GIMP/Photoshop), which breaks the workflow.

• Open-Source Precedent:
  GIMP (Tools → Paths → Shape Tools), Krita (Vector Layers), and Inkscape offer similar features.

Note: This is a new UI feature, not a bug or enhancement of existing tools.**

No files, tests, or entry points are named. Start by reviewing the darkroom module's existing freehand mask and path tools, then define how geometric shapes integrate with masking and retouching workflows. Done means the Draw Shape tool supports rectangles, circles, ellipses, lines, polygons, and point placement with the requested precision behavior.