Persist the tree view zoom level across Notepad++ sessions

[leoshone]

Problem

The JSON tree already has a zoom slider (80%..250%, plus Ctrl+MouseWheel over
the panel), but the chosen level is not remembered: after restarting Notepad++
the tree is always back at 100%.

Steps to reproduce

1. Open a .json file and show the JSON Viewer panel.
2. Drag the zoom slider to 150% (or Ctrl+wheel).
3. Restart Notepad++ and show the panel again.
4. The tree renders at 100% while the slider thumb and the percent label still
   say 150%, until the slider is touched again.

Cause

Setting had no field for the zoom level, so ProfileSetting::GetSettings()
and SetSettings() had nothing to read or write. The level only ever lived in
the trackbar control.

Fix

• Setting::nTreeZoom (default 100) plus a new [Others] TREE_ZOOM key in
  JSONViewer.ini, read and written exactly like the neighbouring keys.
• JsonViewDlg::PersistZoom() writes the value when it changes.
• WM_INITDIALOG re-applies the level that was read from the ini.
• While the slider thumb is being dragged (TB_THUMBTRACK) nothing is written,
  so one gesture produces one write instead of one per pixel.

Notes

• Purely additive: no existing ini key renamed or removed, no default behaviour
  changed, no resource or UI change.
• An existing JSONViewer.ini simply has no TREE_ZOOM key and falls back to
  100%.

Tests

tests/UnitTest/ProfileTest.cpp

• TreeZoom_RoundTrip - default is 100%, and 80/100/150/200/250 survive a
  write/read cycle.
• SetSettings_Positive - extended to cover the new field.