Add base path support for opencode server

[incognitojam]

What does this PR do?

Adds base path support for server routes and proxying, plus CLI/config options and normalization.

How did you verify your code works?

bun turbo typecheck

[github-actions]

Hey! Your PR title Add base path support for opencode server 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.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

feat: base path support (PR #7625)
#7625

This appears to be directly related to the current PR #9528. PR #7625 already addresses base path support, which is the core feature of the current PR. These likely need to be reconciled - either PR #7625 was previously closed/abandoned and this is a fresh implementation, or they may be duplicating the same work.

[incognitojam]

Sorry, didn't mean to open it here.