feat(server): support OPENCODE_WEB_URL for local frontend serving

[sjawhar]

Issue for this PR

Closes #12445
Related: #8549, #11981, #16787

Type of change

• New feature

What does this PR do?

Adds OPENCODE_WEB_URL environment variable to override the default app.opencode.ai proxy in the server's catch-all route. Two modes:

1. File path (OPENCODE_WEB_URL=./packages/app/dist) — serves the built frontend directly from disk with proper MIME types and SPA fallback for client-side routes. Returns 404 for unmatched API-prefixed paths instead of attempting new URL() on a file path.

2. HTTP URL (OPENCODE_WEB_URL=https://internal-host.corp/opencode-app) — proxies to a custom host instead of app.opencode.ai, with correct Host header rewriting.

Without the variable, behavior is unchanged (proxies to app.opencode.ai).

The catch-all route sits at the end of the Hono chain, after all API routes. It only handles requests that don't match any API endpoint — static assets, SPA routes, etc.

How did you verify your code works?

• Built the app (bun run build in packages/app), ran OPENCODE_WEB_URL=../app/dist bun run --conditions=browser ./src/index.ts serve --port 8080 from packages/opencode — app loads, all API endpoints work, SPA routing works for client-side routes.
• Verified without the env var — behavior unchanged, proxies to app.opencode.ai as before.
• bun typecheck passes.

Screenshots / recordings

N/A — backend-only change, no UI modifications.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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

Based on my search, I found 2 potentially related PRs (excluding the current PR #17104):

1. PR feat(server): embed web UI assets in binary and serve locally #15721 - "feat(server): embed web UI assets in binary and serve locally"

   • Related because it also addresses serving frontend locally, which overlaps with the file path mode of this PR

2. PR feat: add OPENCODE_APP_URL for custom web app proxy #12446 - "feat: add OPENCODE_APP_URL for custom web app proxy"

   • Related because it also provides URL customization for the web app proxy, similar to the HTTP URL mode of this PR

These PRs may have alternative or overlapping approaches to solving the same problem. You should review them to ensure this PR doesn't duplicate existing functionality.

[github-actions]

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