fix(auth): respect server username in clients

[nexxeln]

summary

• centralize OpenCode server basic auth header creation
• use configured server usernames for attach, run attach, ACP, TUI worker, and plugin clients
• add attach --username/-u and coverage for auth header precedence

Closes #25113

testing

• bun test test/server-auth.test.ts
• bun typecheck (packages/core)
• bun typecheck (packages/opencode)
• bun turbo typecheck

[nexxeln]

/review

[github-actions]

lgtm

[OpeOginni]

@nexxeln based on this PR, could you take a look at this one #25600, it adds the --username option as well to the run --attach command.

Needed as I added that to the documentation update, a while ago which Aiden merged in, but didn't approve the actual logic PR, but your PR does it better.