fix: MD5→SHA-256, shellcheck warnings, and broken doc links

config_service.py: replace MD5 with SHA-256 for config change detection;
same semantics (equality comparison), no stored hashes affected.

Shell scripts — shellcheck warnings:
- diagnose_web_interface.sh: remove useless cat (SC2002)
- dev_plugin_setup.sh: restructure A&&B||C into if/then (SC2015)
- fix_assets_permissions.sh: remove unused REAL_HOME block (SC2034)
- install_web_service.sh: remove unused USER_HOME assignment (SC2034)
- diagnose_web_ui.sh: remove unused SUDO assignments (SC2034)
- diagnose_plugin_permissions.sh: remove unused BLUE color var (SC2034)
- first_time_install.sh: remove unused CLEAR var, PACKAGE_NAME
  assignment, and replace loop variable with _ (SC2034)

docs/PLUGIN_ARCHITECTURE_SPEC.md: fix 10 broken TOC anchor links to
include section numbers matching the actual headings (MD051).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/diagnose_web_ui.sh

@@ -16,11 +16,8 @@ YELLOW='\033[1;33m'
 NC='\033[0m' # No Color
 
 # Check if running as root or with sudo
-if [ "$EUID" -ne 0 ]; then 
+if [ "$EUID" -ne 0 ]; then
     echo -e "${YELLOW}Warning: Some checks require sudo. Running what we can...${NC}"
-    SUDO=""
-else
-    SUDO=""
 fi
 
 PROJECT_DIR="${HOME}/LEDMatrix"