Remove unused usePlatformService composable and document mixin architecture

- Delete unused usePlatformService.ts file
- Create architecture-decisions.md documenting mixin choice over composables
- Update README.md with architecture decision reference and usage guidance
- Document rationale: performance, consistency, developer experience
- Maintain established class-based component pattern with vue-facing-decorator

.cursor/rules/development_guide.mdc

@@ -3,10 +3,12 @@ description:
 globs: 
 alwaysApply: true
 ---
-use system date command to timestamp all interactions with accurate date and time
-python script files must always have a blank line
-remove whitespace at the end of lines
-never git add or commit for me.  always preview changes and commit message to use and allow me to copy and paste
+✅ use system date command to timestamp all interactions with accurate date and time
+✅ python script files must always have a blank line at their end
+✅ remove whitespace at the end of lines
+✅ use npm run lint-fix to check for warnings
+
+always preview changes and commit message to use and allow me to copy and paste
 ✅ Preferred Commit Message Format
 
     Short summary in the first line (concise and high-level).