app: refactor server management backend

[Brendonovich]

Refactors the server management backend (no ui changes) to do a few things:

• Hoist desktop- and web-specific logic outside of app
  • Default url has been hoisted as desktop always supplies its own, and the other calculation logic was web-specific
  • Servers that aren't managed by app (sidecar, ssh, etc.) can be provided externally, to make data flow nicer
• app is now aware of plain http connections as well as other types (sidecar, ssh, WSL) which all use the same http type internally (which has username + password)
• Sidecar password is no longer exposed via __OPENCODE.serverPassword, instead it's injected as the password for a custom server
• Usage of createOpencodeClient has been replaced by a wrapper that takes a ServerConnection to ensure that SDK clients are always created with the server's username + password

Still TODO:

• Convert context/server to only use ServerConnection
• Test remote connections

[github-actions]

Hey! Your PR title app: refactor server management backend doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.