refactor: collect extra partitions from module root

- Add collect_partition_from_modules() for direct partition collection
- Remove odm from builtin_parts (now via EXTRA_PARTS)
- Extra partitions: <module>/<part>/ instead of <module>/system/<part>/
- Builtin partitions (vendor/product/system_ext) unchanged for compatibility

webui/src/App.svelte

@@ -543,7 +543,7 @@
               id="partitions-input"
               type="text"
               bind:value={partitionInput}
-              placeholder="mi_ext,my_stock"
+              placeholder="eg. odm,mi_ext,my_stock"
             />
           </div>