feat: base path support (rebased from #7625)

[nick-the-nuke]

Summary

• Rebases base-path support from PR feat: base path support #7625 onto v1.2.13
• Adds --base-path CLI option, OPENCODE_BASE_PATH env var, and server.basePath config
• Rewrites HTML/JS/CSS responses at runtime to include the base path
• Injects window.__OPENCODE_BASE_PATH__ into frontend for SolidJS Router compatibility

What this PR does

This is a rebase of @prokube's excellent work in #7625 onto the latest release tag (v1.2.13), resolving conflicts that arose from 275+ commits of drift.

Key features:

• Configurable base path via CLI (--base-path), env var (OPENCODE_BASE_PATH), or config (server.basePath)
• Runtime rewriting of proxied frontend assets to work under any URL prefix
• History API wrapper to prepend base path to pushState/replaceState calls
• Vite base path function patching for dynamic asset loading
• Health endpoint now returns basePath for clients to discover the configuration

Testing

# Start server with base path
opencode web --base-path /myapp

# Or via environment variable
OPENCODE_BASE_PATH=/myapp opencode web

Access at http://localhost:4096/myapp/

Related

• Original PR: feat: base path support #7625
• Original issue: [FEATURE]: Base path / prefix routing support #7624

🤖 Generated with Claude Code

[github-actions]

Thanks for your contribution!

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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

Potential Duplicate Found:

• PR feat: base path support #7625: feat: base path support (feat: base path support #7625)
  • This is the original PR that the current PR feat: base path support (rebased from #7625) #12333 is rebasing from. The current PR explicitly states it's a rebase of feat: base path support #7625 onto v1.2.13 to resolve conflicts.

The current PR #12333 is not a duplicate but rather a modernized rebase of the original work. If #7625 is still open, you may want to close it in favor of this newer version once #12333 is merged.