Device-abstraction refactor: HAL + per-board folders + responsive UI

Replaces the build-flag-driven #ifdef sprawl (~30 blocks across 6 files)
with a small HAL in firmware/src/hal/ and per-board folders under
firmware/src/boards/. Shared code (main.cpp, ui.cpp, splash.cpp) no
longer contains a single `#ifdef BOARD_*` — optional features are
guarded by BoardCaps (runtime) and BOARD_HAS_* macros (compile-time,
inside the board's own files).

Why: lets community contributors port to new ESP32 + AMOLED + touch
combos by dropping in a boards/<name>/ folder + a PlatformIO env,
without touching shared files. See docs/porting/adding-a-board.md.

Highlights:

- New HAL: display_hal, touch_hal, input_hal, power_hal, imu_hal,
  board_caps. Each board provides display.cpp, touch.cpp, input.cpp,
  power.cpp, imu.cpp, caps.cpp, board_init.cpp + private hardware
  drivers (e.g. io_expander.{h,cpp} on AMOLED-1.8).
- PlatformIO build_src_filter selects each board's folder per env.
- ui.cpp picks fonts and layout from board_caps() via compute_layout()
  with screen-height breakpoints (>= 460 → large, else compact).
- splash.cpp computes CELL = min(W,H)/20 — responsive instead of two
  hardcoded values.
- idle.cpp (from #24) rewired through display_hal + power_hal — no
  longer depends on the deleted display_cfg.h / power.h.
- power_hal gains power_hal_is_vbus_in() for idle's
  IDLE_SLEEP_WHEN_CHARGING gate.
- boards/template/ + docs/porting/{adding-a-board,hal-contract,
  capability-flags}.md to bootstrap new ports.
- display_cfg.h, power.{h,cpp}, imu.{h,cpp}, io_expander.{h,cpp}
  deleted from src/ root (moved into boards/<name>/ or hal/).

Verification: both `pio run -e waveshare_amoled_216` and
`pio run -e waveshare_amoled_18` succeed unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -25,10 +25,18 @@ While the splash is up, the middle button cycles animations instead of screens.
 
 ## Hardware
 
-- [Waveshare ESP32-S3-Touch-AMOLED-2.16](https://www.waveshare.com/esp32-s3-touch-amoled-2.16.htm?&aff_id=149786) - ESP32-S3R8, 2.16" 480×480 AMOLED (CO5300 QSPI), CST9220 cap touch, AXP2101 PMU + Li-Po battery, QMI8658 IMU
+Two boards are supported out of the box:
+
+- [Waveshare ESP32-S3-Touch-AMOLED-2.16](https://www.waveshare.com/esp32-s3-touch-amoled-2.16.htm?&aff_id=149786) — ESP32-S3R8, 2.16" 480×480 AMOLED (CO5300 QSPI), CST9220 cap touch, AXP2101 PMU + Li-Po battery, QMI8658 IMU. Three side buttons, IMU auto-rotation. Build env: `waveshare_amoled_216`.
+- [Waveshare ESP32-S3-Touch-AMOLED-1.8](https://www.waveshare.com/esp32-s3-touch-amoled-1.8.htm) — ESP32-S3R8, 1.8" 368×448 portrait AMOLED (SH8601 QSPI), FT3168 cap touch, AXP2101 PMU, QMI8658 IMU, XCA9554 IO expander, 16 MB flash. Two buttons (BOOT + PWR), fixed orientation. Build env: `waveshare_amoled_18`.
+
+Plus per board:
+
 - USB-C cable for flashing firmware and charging
 - 3.7V Li-Po battery (MX1.25 2-pin connector, optional)
 
+**Porting to another board:** the firmware is a thin HAL with per-board folders under `firmware/src/boards/`. Drop in a new folder and a new PlatformIO env — `main.cpp`, `ui.cpp`, and `splash.cpp` never need to change. See [`docs/porting/adding-a-board.md`](docs/porting/adding-a-board.md) for the walk-through and [`docs/porting/hal-contract.md`](docs/porting/hal-contract.md) for the interfaces a port must implement.
+
 ## Prerequisites
 
 - Linux (tested on Ubuntu) or macOS
@@ -188,6 +196,31 @@ lv_font_conv --font assets/DejaVuSansMono.ttf \
 
 Without these patches, fonts compile but render as invisible.
 
+### CJK support
+
+`firmware/src/font_cjk_16.c` covers the full CJK Unified Ideographs basic
+block (U+4E00–U+9FFF, ~20k glyphs) plus ASCII, CJK punctuation, and
+halfwidth/fullwidth forms. Generated from [Noto Sans CJK SC](https://github.com/notofonts/noto-cjk)
+(SIL OFL 1.1) at 16px, 2bpp:
+
+```bash
+lv_font_conv --font NotoSansCJKsc-Regular.otf --size 16 --bpp 2 \
+  --no-compress --format lvgl --lv-include 'lvgl.h' \
+  -r '0x20-0x7E,0xB7,0x2014,0x2018-0x2019,0x201C-0x201D,0x2026,0x3000-0x303F,0x4E00-0x9FFF,0xFF00-0xFFEF' \
+  -o firmware/src/font_cjk_16.c
+```
+
+Then apply the four LVGL 9 patches above. Because the font has >65k of
+glyph bitmap data, the build needs `-DLV_FONT_FMT_TXT_LARGE=1` in
+`platformio.ini` build flags so font descriptor offsets switch from
+16-bit to 32-bit.
+
+The CJK font is used for the Activity screen's user-prompt row and todo
+content rows. The headline (28pt Styrene B) and titles stay ASCII-only
+to preserve the brand font — Chinese text in those slots renders as
+empty boxes. Add a `font_cjk_28.c` if full coverage is needed (~1MB
+more flash).
+
 ## Converting Lucide icons
 
 The UI uses a small set of [Lucide](https://lucide.dev) icons (bluetooth + battery states) converted to RGB565 / RGB565A8 C arrays for LVGL.