Add scr_main skeleton for ITGC-100 800x480 display

[Oratorium-No8]

Implements the main screen (scr_main) widget structure in the Windows Simulator per docs/ITGC100_widget_structure.md:

• create_header_bar(): shared header with mem_selector (◀ 001 ▶), START/STOP buttons, gear icon; btn_back hidden on main screen
• create_ch_cell(): 16-channel 4x4 grid (132x98 each) with accent_bar, result label, and X/Y voltage labels
• update_ch_cell(): runtime update function for CH_INACTIVE/OK/NG states
• create_scr_main(): right panel with trend chart (lv_chart LINE 10pts), pie canvas placeholder (100x100), OK/NG count cards, ellipse a/b labels, and counter reset button; status bar at y=462 h=18

Also enables LV_FONT_MONTSERRAT_10/12/16 in lv_conf.h (required by the widget size spec).

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Summary by cubic

Add the scr_main UI skeleton for the ITGC-100 800x480 Windows simulator. Provides the header, 16-channel grid, right panel placeholders, and a status bar; enables required Montserrat fonts.

• New Features
  • Shared header: memory selector (◀ 001 ▶), START/STOP, gear; back button hidden on main.
  • 16-channel 4x4 grid (132x98) with accent bar, result, and X/Y voltage labels; update_ch_cell() handles CH_INACTIVE/OK/NG and value updates.
  • Right panel: trend line chart (10 points), pie canvas placeholder (100x100), OK/NG count cards, ellipse a/b labels, counter reset.
  • Bottom status bar (y=462, h=18) with datetime and latest NG.
  • Montserrat fonts 10/12/16 enabled in lv_conf.h to match the spec.

^{Written for commit c313c4f. Summary will update on new commits.}

[cubic-dev-ai]

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