Add Libra Colour / Elipsa 2E support, fix config persistence, UTF-8 rendering, and page-turn buttons

[systematiccaos]

Device support:

• Add touch quirks for Kobo Libra Colour (monza/390) and Elipsa 2E (condor/389) with APP_TOUCH_MIRROR_Y=1
• Query actual framebuffer dimensions at runtime in diagnostics instead of hardcoding 1072x1448

Config persistence:

• Add fflush()+fsync() before fclose() in config_save() to ensure data reaches persistent storage before the forced reboot
• Guard persist_session() and save_session() against writing empty session data that would overwrite a valid config.ini
• Add double sync with 2s delay before reboot in run.sh to let the eMMC write-back cache drain (prevents FAT32 corruption / FSCK files)
• Add diagnostic logging throughout bootstrap, config, and path code

UTF-8 rendering:

• Rework sanitize_bitmap_text() → sanitize_display_text(): when OT fonts are loaded, pass UTF-8 through to FBInk's FreeType/HarfBuzz renderer instead of replacing all non-ASCII with '?'. German umlauts, accented characters, CJK, etc. now render correctly. The ASCII-only fallback is kept for the bitmap VGA font path.
• Expose font_ot_enabled() so callers can query the rendering mode

Physical page-turn buttons:

• Accept any EV_KEY device during input scan (the Libra Colour's gpio-keys was silently dropped because it didn't match the narrow key code allowlist)
• Accept key_press events from all key-capable devices, not just the first one found
• Map KEY_F23 (193) / KEY_F24 (194) for Libra Colour, plus KEY_PAGEUP/DOWN, KEY_NEXTSONG/PREVIOUSSONG, KEY_BACK/FORWARD for other models
• Route page-turn buttons to feed (next/prev page), thread (scroll), and settings (scroll) views