Add HIL mode visual indicator on device screen

[Amperstrand]

Summary

When the device is running HIL-enabled firmware, there is no visual indication on the device itself. A developer (or user) picking up the device has no way to tell it's running test firmware versus a normal build.

Why This Matters

• HIL firmware exposes TEST_MNEMONIC over the ST-Link UART
• The device auto-enables USB host and forces regtest network
• A HIL build accidentally used as a daily driver would behave unexpectedly
• Industry practice: Trezor and Coldcard both show visual state indicators for bootloader/firmware mode

Suggested Approach

A small, non-intrusive indicator. Options:

1. Status bar badge: Show "TEST" in the status bar when platform.hil_test_mode is True
2. LED pattern: Blink an LED in a distinctive pattern during HIL mode (the disco board has 4 LEDs)
3. Startup screen: Show a brief "HIL Test Mode" splash during boot

Option 1 is probably best — always visible, minimal code, and matches how the device already shows network mode and other state.

Resolution Criteria

• Device shows a visible indicator when HIL mode is active
• Indicator is present during normal operation (not just boot)
• Indicator does not block or interfere with normal device interaction
• Zero impact when hil_test_mode is False (no code path executed)
• Indicator is not annoying during active test development

Priority

Low — defense-in-depth safety feature. The existing HIL_ENABLED flag prevents accidental HIL inclusion.