fix(windows-update): preserve repeated distribution keys when requesting latest downloads

[HaloWang]

Summary

This fixes the app update request used by desktop clients when querying /distributions/latest.

The app currently percent-encodes the entire repeated key= query string, so instead of sending:

/distributions/latest?key=winZipGR&key=winHF&key=winAF&key=winGR...

it sends:

/distributions/latest?key%3DwinZipGR%26key%3DwinHF%26key%3DwinAF%26key%3DwinGR...

Because of that, the backend does not receive any key filters and returns all platform distributions.
The client then picks the highest-build *GR asset from the full response, which can incorrectly resolve to Android APKs on Windows.

Root Cause

In lib/store/app.dart, _getLatestVersionInfo() encodes each key correctly, but then encodes the full joined query string a second time:

final queryParts = keys.map((key) => 'key=${Uri.encodeComponent(key)}').toList();
String queryString = queryParts.join('&');
queryString = Uri.encodeComponent(queryString);

That second Uri.encodeComponent(queryString) is the bug.

Expected vs Actual

If Apple made this mistake, what would we expect?

A macOS client should request only macOS distribution keys, receive only macOS assets, and when a direct installer is available, open that installer directly. If no direct installer is available, it should fall back to the official download website.

What actually happens when this bug exists?

The malformed request drops the platform filter. The backend responds with assets for every platform. The client then selects a highest-build GitHub Release asset from that mixed list, so a Windows x64 user can be sent to the Android APK instead of a Windows installer or zip.

Fix

Stop encoding the full joined query string. Keep only the per-key encoding:

final queryParts = keys.map((key) => 'key=${Uri.encodeComponent(key)}').toList();
final queryString = queryParts.join('&');

User Impact

• Windows x64 update prompts will resolve to Windows assets again
• When a direct installer exists, the update action can open it directly
• If direct download cannot be resolved, the existing fallback to the official download page remains intact