refactor(steami_screen): move constants to dedicated const.py file.

[MatteoCnda1]

Summary

Refactor steami_screen module by extracting all constants from device.py into a dedicated const.py file, aligning with the project convention of having a const.py per module (as in apds9960, hts221, bq27441, etc.).

Closes #358

Context

The project convention recommends having a const.py file in each module to group constants. In steami_screen, constants such as colors and FACES were previously defined directly in device.py, making the file more cluttered and diverging from the established structure used elsewhere in the repository.

Changes

New file

• lib/steami_screen/steami_screen/const.py — centralizes all module constants

Constants moved from device.py to const.py

• Colors (RGB tuples): BLACK, DARK, GRAY, LIGHT, WHITE, GREEN, RED, BLUE, YELLOW
• Bitmaps: FACES dictionary (8x8 pixel-art expressions)

New constants extracted from inline magic numbers

• STEAMI_CHAR_W / STEAMI_CHAR_H — framebuf font dimensions (previously inline class attributes)
• STEAMI_DEFAULT_WIDTH / STEAMI_DEFAULT_HEIGHT — default screen size (previously hardcoded 128)
• STEAMI_GAUGE_START_ANGLE / STEAMI_GAUGE_SWEEP — gauge widget angles
• STEAMI_GRAPH_MARGIN, STEAMI_GRAPH_X_OFFSET, STEAMI_GRAPH_Y, STEAMI_GRAPH_HEIGHT, STEAMI_GRAPH_VALUE_Y, STEAMI_GRAPH_DASH, STEAMI_GRAPH_GAP — graph widget layout
• GRID_DARK — graph grid color (previously inline (51, 51, 51) tuple)

Updated files

• device.py — imports constants from const.py, no functional changes
• __init__.py — colors and FACES are now re-exported directly from const.py (single source of truth)

Motivation

• Improve readability and maintainability of device.py
• Reduce file size and responsibility of device.py
• Align with project conventions (const.py per module)
• Make constants easier to locate and reuse across the module

API compatibility

No public API breakage.

• Screen.CHAR_W and Screen.CHAR_H remain exposed as class attributes for backward compatibility
• All colors and FACES remain importable from the top-level package: from steami_screen import WHITE, FACES
• All examples in lib/steami_screen/examples/ work without modification

Validation

• ruff check passes
• python -m pytest tests/ -k mock -v passes
• No public API breakage
• Hardware test on STeaMi board: comfort_bar_demo.py runs successfully via mpremote

Notes

This is a pure refactor with no functional changes. The apds9960 module's const.py was used as the reference for naming convention (STEAMI_* prefix for module-specific constants, short names for shared color tuples).