Feature/viewport grid overlay - #16

Adds an optional 2D viewport grid overlay to the realsense-viewer,
compiled under the BUILD_VIEWPORT_GRID_OVERLAY CMake flag (default OFF).

Feature summary
- Grid lines rendered over each 2D stream viewport using OpenGL
  (glBegin/GL_LINES) with glPushAttrib(GL_CURRENT_BIT) to prevent
  color-state bleed into PiP thumbnails
- Two new toggle buttons in each stream header:
  - Grid icon (fa-th ): show/hide the grid overlay
  - Info icon: show/hide viewport metadata overlay
  Both buttons start OFF at every launch (state is not persisted)
- Per-stream grid config: horizontal lines [1-5], vertical lines [1-5],
  line width (>=1), RGB color [0-255 each]; out-of-range values fall
  back to defaults (not clamped)

Configuration (realsense-config.json)
- Grid settings stored under viewer_model.grid.* using dot-path nested
  JSON API (get_nested / set_nested / set_nested_default)
- Defaults are written only in two cases:
  1. New file: config_file tracks _is_new_file (set in the
     file-based constructor when load_from_file returns nothing);
     ux_window writes defaults on startup when is_new_file() is true
  2. Restore Defaults button: viewer.cpp injects grid defaults into
     temp_cfg immediately after the reset, so they are saved on OK/Apply
- Existing configs without the grid section are left untouched

Files changed
- CMake/lrs_options.cmake: add BUILD_VIEWPORT_GRID_OVERLAY option (OFF)
- common/device-model.h: configurations::viewer::viewport_grid namespace
  with the six JSON key constants
- common/textual-icons.h: textual_icons::grid icon (, fa-th)
- common/rs-config.h / rs-config.cpp: is_new_file() accessor backed by
  _is_new_file member set in the filename constructor
- common/stream-model.h / stream-model.cpp: grid state members, toggle
  buttons, draw_2d_grid() helper; constructor reads grid config from JSON
- common/ux-window.cpp: write grid defaults on new-file startup
- common/viewer.cpp: write grid defaults when Restore Defaults is pressed
- tools/realsense-viewer/CMakeLists.txt: target_compile_definitions for
  BUILD_VIEWPORT_GRID_OVERLAY

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>