Session mode: persistent annotations, measurement export & composite screenshot

[pgivel]

Summary

Add a session mode that turns Screen Ruler from a quick-check tool into a persistent annotation workspace for UI validation.

Entering / exiting session mode

• Press Tab while in normal (quick) mode to enter session mode.
• In session mode, Esc quits (with a prompt/confirmation if annotations exist — TBD).
• Normal mode (current behavior) is unchanged: click copies measurement and quits.

Interaction changes in session mode

• Click places a persistent annotation (marker + measurement label) on screen, using the current mode's measurement. The annotation stays visible.
• Ctrl+Z / Z undoes the last annotation.
• 1 / 2 / 3 / … switches measurement mode (crosshair, drag rect, container detection, and future modes).
• Annotations accumulate — multiple measurements can coexist on screen.

Annotation model

Add a Python-side annotation list exposed to QML (e.g. as a QAbstractListModel or a simple list property). Each annotation stores:

• Mode that produced it
• Geometry (rect, point(s), or measurement bounds)
• Measurement text
• Optional: color info, guide reference

QML renders annotations via a Repeater over this model using existing visual components (SelectionOutline, MeasurementLabel, SnappedPointerMarker) instantiated per annotation.

Export: Ctrl+C — Markdown measurement list

Copy all current annotations as a Markdown-formatted list to the clipboard.

Example output:

- Crosshair @ (412, 230): 48 × 32 px
- Container @ (100, 80): 320 × 200 px
- Point-to-point: A(10, 20) → B(150, 45) — 143.4 px (Δx=140, Δy=25)

Export: Ctrl+Shift+C — Composite screenshot

1. Enter a rectangle-drag selection (click & drag to choose export region).
2. Render the selected region of the original screenshot with annotations overlaid into a composite image.
3. Copy the resulting image to the clipboard.
4. (Optional) Also support Ctrl+S to save to a file with a dialog.

This serves the "proof of correctness/error" use case — the user can paste the annotated screenshot directly into a ticket, Slack, etc.

Design notes

• The current quick mode must remain the default — session mode is opt-in.
• A visual indicator (e.g. small badge or tinted status) should signal that session mode is active.
• Session mode is the prerequisite for annotation comparison, linking, and other future collaborative features.