v0.9.2: update check tunnels through proxy + one-click asset download (#15 follow-up)

@zula-editor reported on issue #15 that the Check-for-updates button
was returning HTTP 403 on their ISP — classic GitHub
unauthenticated-API rate limit (60/hour per IP) on a shared NAT IP.
They also asked for the update to actually be downloadable from the
app, not just a page link.

Both addressed:

=== Route update check through our own proxy when running ===

New mhrv_rs::update_check::Route enum:
  - Direct: straight rustls to api.github.com (existing behavior)
  - Proxy { host, port }: HTTP CONNECT through our local HTTP proxy
    listener → MITM → Apps Script → api.github.com.

When the proxy is running, the UI automatically picks Proxy. From
GitHub's POV the request now comes from Apps Script's IP range (a
Google datacenter) — completely different rate-limit bucket from the
user's ISP IP, AND works even if GitHub is blocked on their network.

Routing over proxy means the MITM leaf for api.github.com has to be
trusted in the update_check's TLS config. build_root_store() now
conditionally adds our own CA cert from data_dir::ca_cert_path() to
the webpki roots when Route::Proxy is in use. Direct path is
unchanged.

=== Download button ===

The UpdateCheck::UpdateAvailable variant now carries an optional
ReleaseAsset { name, download_url, size_bytes } picked by
pick_asset_for_platform() from the GitHub API's assets[] array.
Preference list per (OS, arch):
  - macOS arm64 → mhrv-rs-macos-arm64-app.zip, else tar.gz
  - macOS amd64 → mhrv-rs-macos-amd64-app.zip, else tar.gz
  - Windows → mhrv-rs-windows-amd64.zip
  - Linux aarch64 → mhrv-rs-linux-arm64.tar.gz
  - Linux armv7 → mhrv-rs-raspbian-armhf.tar.gz
  - Linux x86_64 → mhrv-rs-linux-amd64.tar.gz

UI: when an update is available AND we have an asset, the transient
status line grows an accent-blue 'Download X.Y MB' button. Clicking
fires Cmd::DownloadUpdate, which pipes the asset through the same
Route (proxy if running, direct otherwise), writes it to
UserDirs::download_dir() (~/Downloads on most systems), and shows a
'show in folder' button that opens Finder / Explorer / xdg-open on
the containing directory.

Three new unit tests for asset-picking. The gated live test now
takes a Route argument (Direct) so it keeps working across the API
shape change. 49 tests pass.

Also refreshed in-repo releases/ archives to v0.9.1 alongside.

Author: therealaleph

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "mhrv-rs"
-version = "0.9.1"
+version = "0.9.2"
 edition = "2021"
 description = "Rust port of MasterHttpRelayVPN -- DPI bypass via Google Apps Script relay with domain fronting"
 license = "MIT"

Cargo.toml

@@ -2,7 +2,7 @@
 
 This folder contains the prebuilt binaries from the latest release, committed directly to the repository for users who cannot reach the GitHub Releases page.
 
-Current version: **v0.9.0**
+Current version: **v0.9.1**
 
 | File | Platform | Contents |
 |---|---|---|
@@ -50,7 +50,7 @@ See the [main README](../README.md) for full setup (Apps Script deployment, conf
 
 این پوشه شامل فایل‌های آخرین نسخه است و مستقیماً در ریپو قرار گرفته برای کاربرانی که به صفحهٔ GitHub Releases دسترسی ندارند.
 
-نسخهٔ فعلی: **v0.9.0**
+نسخهٔ فعلی: **v0.9.1**
 
 ### دانلود از طریق ZIP