refactor(firmware): per-panel folder layout + parametrized config.h

Reorganizes the tree so adding a new panel is purely additive — drop in a
new src/panels/{vendor}/v{N}/ folder and a new platformio.ini env block,
no surgery to existing files.

Layout:
  src/                              shared across all panels
  src/panels/waveshare73/v1/        V1 driver, version, README
  data/waveshare73-v1/              LittleFS payload at this panel's size

src/config.h still defines the panel-agnostic bits (NVS keys, color
palette, network, sync-fail border) but EPD_WIDTH / EPD_HEIGHT / pin
assignments now come from each env's -D flags. Strict #error guards in
production builds; native tests get the V1 defaults via UNIT_TEST.

build_src_filter per env picks the right driver:
  waveshare73-v1   main + panels/waveshare73/v1/
  test-display     test_display + panels/waveshare73/v1/
  sim-yellow       sim_border + panels/waveshare73/v1/
  sim-red          sim_border + panels/waveshare73/v1/
  native-test      unchanged

When V2 hardware lands, the diff is a new env block, a new
src/panels/waveshare133/v1/epd_driver.cpp, and regenerated screens at
data/waveshare133-v1/. Existing V1 envs stay frozen — re-flashing old
units remains a one-liner.

scripts/gen_screens.py takes --panel to target the correct
data/{panel}/ subfolder; defaults to waveshare73-v1.

29/29 native tests pass. All four hardware envs build clean.

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

scripts/gen_screens.py

@@ -329,13 +329,20 @@ def save_bin(img, path, preview_path):
     print(f"Preview → {os.path.abspath(preview_path)}")
 
 if __name__ == "__main__":
-    out_dir = os.path.join(os.path.dirname(__file__), "../data")
+    # Per-panel output directory: data/{vendor}-v{N}/. Defaults to the V1
+    # 7.3" panel since that's the only one in production; pass --panel to
+    # target a different one once new panels exist.
+    panel = "waveshare73-v1"
+    if "--panel" in sys.argv:
+        panel = sys.argv[sys.argv.index("--panel") + 1]
+
+    out_dir = os.path.join(os.path.dirname(__file__), f"../data/{panel}")
     os.makedirs(out_dir, exist_ok=True)
 
-    print("Generating AP screen…")
+    print(f"Generating AP screen for {panel}…")
     save_bin(gen_ap(),    f"{out_dir}/ap_bg.bin",    f"{out_dir}/ap_bg_preview.png")
     print()
-    print("Generating setup screen…")
+    print(f"Generating setup screen for {panel}…")
     save_bin(gen_setup(), f"{out_dir}/setup_bg.bin", f"{out_dir}/setup_bg_preview.png")
     print()
     print("QR overlay constants for epd.cpp:")