feat: architecture refactor — core layer, spreadsheet engine, envelope bidding, tests

- Add src/core/ (config, errors, events, http, observability, storage, query)
- Add src/Spreadsheet/ engine with native POI backend and Excel bridge
- Add Envelope C spreadsheet flow and tender bidding envelope modules
- Consolidate components into src/components/, remove legacy Component/ and Hooks/
- Add registration state/validation, auth session guard, profile edit modules
- Add unit tests across axios, redux, bidding, registration, encryption, core
- Add CI workflow, docs (architecture, audit, DSC API spec), keystore example
- Remove tracked keystore binary; ignore scripts.zip

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

android/build.gradle

@@ -1,7 +1,7 @@
 buildscript {
     ext {
         buildToolsVersion = "35.0.0"
-        minSdkVersion = 24
+        minSdkVersion = 26
         compileSdkVersion = 35
         targetSdkVersion = 35
         ndkVersion = "27.1.12297006"