Complete Phase E theming, onboarding, and Windows share integration

Adds theme presets (Toffee/Slate/Evergreen/Lavender) with a "follow system"
mode and high-contrast awareness, a first-run welcome screen, and the
Windows analog of Android's share sheet for a Win32 app: an aerofetch://
protocol handler plus an Explorer "Send to AeroFetch" entry, both routed
through a single-instance lock so a second launch hands its link to the
already-running window instead of opening a duplicate.

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

electron-builder.yml

@@ -2,6 +2,15 @@ appId: com.aerofetch.app
 productName: AeroFetch
 copyright: yt-dlp frontend
 
+# Lets a browser/another app hand AeroFetch a link via aerofetch://download?url=<encoded>
+# (src/main/deeplink.ts) — the closest Windows analog to Android's share-to-app intent.
+# The NSIS installer registers this at install time; main/index.ts's
+# app.setAsDefaultProtocolClient call additionally covers the portable build and dev.
+protocols:
+  - name: AeroFetch
+    schemes:
+      - aerofetch
+
 directories:
   buildResources: build
   output: dist