[FEATURE]: Configurable web UI title to tell multiple instances apart

Feature hasn't been suggested before.

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Describe the enhancement you want to request

I run several opencode web instances at once, one per project. Every browser tab is titled "OpenCode", so I can't tell them apart and have to click through tabs to find the one I want.

packages/app/index.html hardcodes <title>OpenCode</title> and nothing overrides it at runtime, so there is currently no way to distinguish instances.

I'd like to set the title per instance with an env var:

OPENCODE_WEB_UI_TITLE="work: frontend" opencode web

I have a working change that rewrites the <title> in the served HTML, for both the embedded UI and the proxied dev UI, and leaves the title alone when the variable is unset. Happy to open a PR if you're open to this approach.