web: plan/build selector missing when accessing remote instance from mobile Firefox

Summary

Accessing a running OC2 desktop instance remotely (opencode serve / opencode pair, via Mac internal IP) from Firefox on Android never shows the plan/build selector in the web UI composer/header area.

Environment

• opencode version: v2.0.2 (opencode --version)
• OS: Darwin G613393F4F 25.6.0 Darwin Kernel Version 25.6.0: Tue Aug 18 17:51:47 PDT 2026; root:xnu-12377.161.15.700.19~2/RELEASE_ARM64_T6050 arm64
• Terminal: xterm-256color (COLORTERM empty, TERM_PROGRAM empty)
• Shell: /bin/zsh
• Install/channel: latest (local install, v2.0.2 vs skill snapshot 2.0.3)
• Active plugins: none found in config (checked opencode.jsonc, no plugin key; no .opencode/plugin/ files)

Reproduction

1. Start OC2 desktop instance on Mac, expose via opencode serve / opencode pair.
2. From Android phone with Firefox, navigate to Mac internal IP instance URL.
3. Open any session / composer view.
4. Observe composer/header area — plan/build selector is not rendered.

Expected Behavior

Plan/build mode selector should be visible and switchable in the web UI, same as desktop/TUI.

Actual Behavior

Selector is always missing in the remote mobile web view. No error observed; rest of UI loads.

Additional Context

• Client: Firefox on Android, remote access via Mac internal IP.
• Frequency: always reproducible.
• TUI comparison: not checked.
• No workaround tried yet. No relevant logs/config snippets collected yet.