feat(web): ask the desktop app which server to sign in to

[BryanFRD]

Part of #150, the first of three layers.

The desktop shell was built against http://localhost:3001 and nothing else: desktopPlatform passed the address baked in at build time to the Tauri login command, so an installer handed to somebody would only ever talk to a server on their own machine. That is fine while the only way to run the app is to build it yourself, and useless the moment a release carries an installer, which is what the next layer does.

The sign-in screen now asks for the address on the desktop, remembers it, and offers it again next launch. A bare host is accepted: roxy.example.com/ becomes https://roxy.example.com before it reaches the Rust side, since a scheme is the kind of thing nobody types.

The browser build is untouched. Platform.server is undefined there, which is what hides the field, the same way upload and changePassword gate their own surfaces.

Verification

Driven in a browser with the Tauri bridge stubbed, which is what the desktop build actually goes through: the form shows Server, Email and Password in that order, prefilled with the remembered address; typing roxy.example.com/ and signing in calls invoke('login') with server: "https://roxy.example.com" and leaves that in storage for the next launch. A plain browser context still shows Email and Password alone.

[ferrfleet]

Reads correctly. The address is stored only after login succeeds, localStorage failures degrade to the compile-time fallback instead of blocking sign-in, and the browser platform keeps server absent so asks stays false and the extra control is stripped before emit.

One nit inline on the validator wiring.

Nit: on a desktop build's first run the server field prefills http://localhost:3001, the Angular dev default, since nothing overrides ROXYCLOUD_API_URL for the Tauri bundle. That was the hard-coded server before this PR so it is no worse, but as a prefill in a shipped app it reads like a mistake; leaving the field empty and relying on the placeholder would be clearer, once the required validator actually holds.