Fix the Windows port, offer the beta at install, and install WebView2 for web wallpapers

[claude]

Before: on Windows, nothing haru saved survived closing the app. Every path it keeps state in came from XDG_CONFIG_HOME, XDG_DATA_HOME or HOME, and Windows sets none of the three outside MSYS or Git Bash, so the config file, the overrides file and the engine's data directory all resolved to nothing. The assets had nowhere to install and scene wallpapers could not render at all. The login entry put a console window on screen for the whole session, and closing it — the obvious thing to do with a stray black window — killed the wallpaper. Stop and Restart reported a failure while the wallpaper stayed up. Assigning a wallpaper to the second monitor changed both, and at the next login only one monitor came back. The Steam sign-in link opened truncated. A full volume slider started the wallpaper at about 12%, and a frame-rate slider set to "unlimited" quietly capped it at 30. The window itself drew with blur-behind artefacts and tearing. The first kirie install could only fetch the stable release, and web wallpapers could not run on Windows at all.

After: %APPDATA% and %LOCALAPPDATA% are where settings and data go on Windows, with the XDG and macOS rules untouched everywhere else. The login entry is a kirie.vbs that starts the renderer with no window at all, and sweeps up the old kirie.cmd so it cannot put a second renderer up beside it. Stop goes straight to taskkill /F /T, because the renderer's only window is a child of Explorer's desktop and there is nothing for a polite close to talk to. Every screen in the plan is named to the renderer on every platform, not only on Linux. Links open through explorer rather than through cmd, which was reading the first & in a URL as a command separator. Volume and frame rate are converted once, in kirie_volume() and in arguments(), to the 0–128 and --fps=0 the renderer actually documents. On Windows the window is opaque, vsynced and on DX12. The install-kirie prompt can install the beta. When the machine lacks the Microsoft Edge WebView2 Runtime, which kirie now uses for web wallpapers on Windows (kirie #9), haru offers to install it behind Windows' administrator prompt.

This is the single haru PR. The WebView2 work was briefly #4, which shows as merged only because its commit now lives on this branch.

What this changes

Windows port fixes

Nine fixes, all found by reading the Windows port against the Win32 and renderer documentation and by cross-compiling it for the first time. Seven are Windows-only. Two — the volume scale and the render-scale floor — were wrong on every platform. A tenth keeps a newline inside a wallpaper property from splitting the socket command in two.

The one judgement call worth flagging: the login entry now depends on Windows Script Host. It is present and enabled on every stock Windows install, but an administrator can turn it off by policy, and then nothing comes up at login. The alternative is the console window, which is worse and which any user would close.

The window on Windows

The window asked for transparency, which Windows' DX12 and Vulkan swapchains cannot give, so winit fell back to DWM blur-behind and resizes flashed black or white with garbage at the edges. It presented without vsync, which tears, and wgpu took whichever of Vulkan, DX12 or WGL answered first, which depended on the driver. On Windows the window is now opaque, vsynced and DX12 (WGPU_BACKEND still overrides it). It opens at 1120×660 so it fits a 1080p screen at the default 150% scaling. Linux and macOS are unchanged. These are the likeliest causes of the "visual bugs" reported on v0.6.5, but no screenshot has confirmed it.

Install the beta from the first prompt

The install-kirie prompt has an "Install the beta" choice, starting from whatever the updater is set to. Picking it also turns the updater's beta setting on, so the install keeps following betas. The WebKit/Chromium choice is shown only on Linux, the one platform that publishes more than one build. The prompt names the real install folder instead of ~/.local/bin, which was wrong on Windows.

WebView2 for web wallpapers

• The install-kirie prompt offers "Also install WebView2, for web wallpapers" when the runtime is missing.
• Settings › Renderer shows whether the runtime is installed, with an Install WebView2… button.
• Either one downloads Microsoft's Evergreen bootstrapper, and Windows puts up its administrator (UAC) prompt before it runs.
• The download is run as administrator, so haru checks it first: it must carry a valid Authenticode signature from Microsoft Corporation, or it is not run.
• Detection: haru_apply::webview2::installed() reads the runtime's pv value from the registry keys Microsoft's guidance names (per-machine in both registry views, and per-user). An empty value or 0.0.0.0 counts as not installed.
• Install: a PowerShell script runs Get-AuthenticodeSignature, then Start-Process -Verb RunAs -Wait with /silent /install. A declined prompt, a bad signature and a failed installer each come back as their own message.
• If the WebView2 install fails, a good kirie install is kept, and the prompt says web wallpapers will not run yet.

How it was tested

cargo fmt --all, cargo clippy --workspace --all-targets -- -D warnings and cargo test --workspace are clean on Linux: 171 tests. The same clippy run is clean for x86_64-pc-windows-gnu.

Tests that fail without the fix, or pin the new behaviour:

• every_screen_in_the_plan_is_named_to_the_renderer and the_stand_in_screen_is_not_named_to_the_renderer;
• the_volume_reaches_the_renderer_on_its_own_scale;
• the_beta_choice_starts_where_the_updater_is;
• reading the WebView2 version from reg output, including keys left behind by an uninstall;
• the elevation script checking the signature before it elevates, including a path containing a quote.

Nothing here has been run on Windows. Every Windows claim in this PR is read from the documentation and from the renderer's own docs/COMMANDS.md, not observed. To watch first on a real machine:

• the .vbs login entry, taskkill /F /T, and per-screen assignment on two monitors;
• whether the window's visual problems are gone;
• the UAC prompt, the signature check and the bootstrapper's /silent /install.

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