Add DPI28 (2.8") touchscreen display and touch input support

[3rdIteration]

Motivation

• Enable support for the Waveshare 2.8" DPI framebuffer touchscreen so the device can use the physical 480x640 DPI display with a 240x240 native UI and a bottom touch bar.
• Provide direct touchscreen input (taps, touch-bar buttons, and navigation fallback) so screens can accept taps on UI elements and the touch bar.
• Auto-detect hardware and expose a selectable dpi28_240x240 display configuration to simplify switching between desktop and touchscreen modes.

Description

• Added a framebuffer display driver src/seedsigner/hardware/DPI28.py that scales 240x240 UI images to the 480x480 UI region, composes a 160px touch bar, and writes BGRA/RGB565 to /dev/fb0 (with numpy-accelerated and pure-Python fallbacks).
• Added raw touch input reader src/seedsigner/hardware/touch.py and a touch adapter src/seedsigner/hardware/touchbuttons.py that expose a HardwareButtons-compatible interface plus tap detection, touch-bar events, and helpers like get_last_tap_native_coords() and register_buttons().
• Integrated DPI28/touch into the app: src/seedsigner/hardware/displays/display_driver.py now supports dpi28, src/seedsigner/models/settings_definition.py adds dpi28_240x240 to available display options, and src/seedsigner/gui/renderer.py auto-detects framebuffer/touch hardware and sets SEEDSIGNER_DISPLAY/SEEDSIGNER_TOUCH.
• Wired touch behavior into the UI: src/seedsigner/gui/screens/screen.py, seed_screens.py, and tools_screens.py now select the input handler (touch vs GPIO), register button hit boxes, handle direct taps, manage touch-bar label updates via Renderer.set_touch_bar_labels(), and hide/show the touch bar on screens where appropriate.
• Keyboard and helper updates include src/seedsigner/gui/keyboard.py adding get_key_at_screen_coords() to support direct keyboard taps, and src/seedsigner/gui/toast.py uses the correct input handler for dismiss/cancel behavior.

Testing

• No automated tests were executed as part of this change set; the patch adds the new drivers and touch integration but no CI/test runs were performed in this rollout.

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Codex Task