docs: update all READMEs for cross-platform support and add wemo-core docs

- Root README: add Linux x64 + ARM64 download targets, update release
  assets table, document background scheduler differences per OS,
  add wemo-core shared package description, use repo name dibby-wemo-manager
- apps/desktop README: full rewrite covering Windows installer/portable,
  Linux AppImage/.deb/.rpm (x64 + ARM64), all build commands per OS,
  data storage paths per OS, requirements table
- packages/wemo-core: new README documenting all exports, constants,
  helper functions, day number convention, sun sentinel codes
- about.html: fix version string from "2.0" to "2.0.0"

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

apps/desktop/resources/about.html

@@ -104,7 +104,7 @@
     <img src="icon.png" alt="Dibby Wemo Manager" />
     <div class="header-text">
       <h1>Dibby Wemo Manager</h1>
-      <div class="version">Version 2.0</div>
+      <div class="version">Version 2.0.0</div>
     </div>
   </div>
   <p class="desc">