feat(windows): configure webviewInstallMode so the NSIS installer provisions WebView2

[jamesbrink]

Before opening a PR: a maintainer must acknowledge and approve this feature request first.

Use Case

Follow-up split from #418. The NSIS Windows installer now ships — nightly.yml and release-please.yml build, sign, and upload the *-setup.exe, and src-tauri/tauri.conf.json has the bundle.windows.nsis block. That part of #418 is done, so #418 was closed.

One acceptance item from #418 remains: the installer does not ensure the WebView2 runtime is present. tauri.conf.json has no bundle.windows.webviewInstallMode. On a clean Windows machine without the Evergreen WebView2 runtime, the app fails to render after install.

Proposed Behavior

Set bundle.windows.webviewInstallMode in src-tauri/tauri.conf.json so the NSIS installer provisions WebView2 — downloadBootstrapper (small installer, fetches at install time) or embedBootstrapper, per the Tauri Windows installer docs. Verify on a fresh Windows VM with no WebView2 runtime.

Additional Context

• Split from feat(windows): ship an installer (MSI/NSIS) with WebView2 bootstrapper, alongside bare-exe builds #418 (Windows installer), which is now closed as the installer artifact itself ships.
• Windows ARM64 test VM is available for verification.