Add Pipeline tab as guided entry point in GUI

[nikolareljin]

ID: KF-009

The GUI has 6 unordered tabs (Status, Capture, Reconstruct,
Measure, View, Calibrate). New users must discover the correct order
(Capture → Reconstruct → View) on their own. There is no single-click path
from scan to model.

Fix: insert a "Pipeline" tab as the first tab. It shows three numbered steps:

1. Capture — preset selector + Start button
2. Build Model — preset selector + Reconstruct button, with a progress bar
3. View — Open Viewer button, with mesh thumbnail after reconstruction

Each step shows a status indicator (pending / done). Keep all existing tabs
intact for advanced use. The Pipeline tab shares state with the other tabs
via the same StringVar/IntVar fields.

File: src/kinect_forge/gui.py