Fix widget rendering issue on Raspberry Pi OS startup

[Copilot]

This PR fixes a display issue where RedNotebook shows garbled or corrupted text on startup when running on Raspberry Pi OS 64-bit. The text becomes readable only after manually resizing the window.

Root Cause

The issue is caused by a GTK widget layout/rendering timing problem specific to Raspberry Pi hardware. On startup, the widgets are not properly refreshed after the window is shown, leading to corrupted display until a resize event forces a layout recalculation.

Solution

The fix implements a platform-specific workaround that:

1. Detects Raspberry Pi OS by checking /etc/os-release for Pi-specific identifiers and validating ARM architecture
2. Forces widget refresh after window show using GTK's queue_resize() and queue_draw() methods
3. Uses proper timing with GLib.idle_add() to defer the refresh until the main loop starts
4. Applies safely with exception handling and debug logging

# Only runs on detected Pi systems
if _is_raspberry_pi_os():
    logging.info("Applying Raspberry Pi OS widget rendering fix")
    _fix_pi_widget_rendering(self.main_frame, self.text_vbox)

Key Features

• Platform-specific: Only affects Raspberry Pi OS systems, zero impact on other platforms
• Minimal footprint: Only 63 lines of code added, following existing codebase patterns
• Safe implementation: Comprehensive error handling and fallback behavior
• Automatic: No user intervention required, works transparently

The implementation follows the same pattern as the existing KDE WebKit compositing fix, ensuring consistency with the codebase architecture.

Fixes #829.

Warning

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• esm.ubuntu.com
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