feat: refuse matrix settings a Raspberry Pi 5 can't drive

On a Pi 5 the pinned rgbmatrix library drives the panel through the RP1
chip, and that path supports only row address types 0 and 2, parallel 1-3
and the regular / regular-pi1 / classic / adafruit-hat(-pwm) mappings
(Rp1PioConfigSupported in lib/rp1/rp1_pio_backend.cc). For anything else
CreateFromOptions returns NULL; the Python binding doesn't check, so the
display process crashed on its first call into the matrix and systemd
restarted it into the same crash every 10 seconds.

- src/pi5_matrix_support.py: the rule and Pi 5 detection, matching the
  library's /proc/device-tree/model check
- DisplayManager raises before creating the matrix, so it is a logged init
  failure (reported by /api/v3/hardware/status) and fallback mode
- the config API rejects those settings on a Pi 5 when a request sets
  row_address_type, parallel or hardware_mapping
- the Display form offers only row address types 0 and 2 on a Pi 5, and
  warns when a stored value can't be used
- CLAUDE.md: re-check the rule whenever the submodule is bumped

Review feedback on #586.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/pages_v3.py

@@ -14,6 +14,7 @@ _SAFE_WIDGET_NAME_RE = re.compile(r'^[a-zA-Z0-9_-]{1,64}$')
 _SAFE_WIDGET_SCRIPT_RE = re.compile(r'^[a-zA-Z0-9_-]{1,64}\.js$')
 from src.web_interface.secret_helpers import mask_secret_fields
 from src.common.path_safety import resolve_under, safe_path_component
+from src.pi5_matrix_support import is_raspberry_pi_5
 from web_interface import widget_bundle
 
 logger = logging.getLogger(__name__)
@@ -544,7 +545,8 @@ def _load_display_partial():
         if pages_v3.config_manager:
             main_config = pages_v3.config_manager.load_config()
             return render_template('v3/partials/display.html',
-                                 main_config=main_config)
+                                 main_config=main_config,
+                                 is_pi5=is_raspberry_pi_5())
     except Exception as e:
         logger.error("Error loading partial", exc_info=True)
         return "Error loading partial", 500