Description

During a STM32 (Hapax) firmware self-update, the progress was drawn on top of
the menu — an Update progress: NN % text line plus a bitmap slider strip in
an info box — while the ESP32 update (#555) already renders on a clean dedicated
screen.

This change gives the self-update the same treatment. When bl_flash_app()
starts, it draws a dedicated full-screen progress view:

       Firmware Update
  ──────────────────────
     Do not power off
           45%
  [████████░░░░░░░░░░]

• The screen is cleared and a title + "Do not power off" hint are shown.
• During the multi-second erase phase a centered "Erasing flash…" status is
  shown on the same dedicated screen (replacing the old hourglass overlay).
• A clean framed progress bar (outline + fill) renders with a centered
  percentage, on its own screen — no menu underneath.
• The bar redraws only when the integer percentage changes, so the flash
  chunks don't trigger a screen flush each and slow the update.
• The completion dialog ("Update complete! / Device will reboot" / "Update
  failed!", press OK) is unchanged.

The progress-percentage math is extracted into a pure, host-testable helper
fw_update_progress_percent() (m1_csrc/m1_fw_progress.h).

fw_gui_progress_update() is intentionally left in place — it is still used by
the ESP32 backup-flash path (m1_esp32_fw_update.c); this change only stops the
STM32 self-update from using it.

Type of Change

• New feature (non-breaking change which adds functionality)

Hardware Requirements

• M1 Device: Main Board REV2.8R

Testing Environment

• Toolchain: CMake + arm-none-eabi-gcc 14.2
• Host tests: GCC + Unity (ctest)

How Has This Been Tested?

• On-device, REV2.8R: ran a STM32 firmware self-update. The progress now
  renders on a clean dedicated screen (title + "Do not power off"), shows
  "Erasing flash…" during erase, then the framed bar fills smoothly, and the
  "Update complete!" reboot dialog appears at the end.
• Host unit tests for the percentage helper (tests/test_fw_progress.c,
  7 cases: start=0, done=100, mid values, total=0 guard, remainder>total
  clamp, real image size). Full suite: 98/98 pass.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (changelog fragment)
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes