fix: stop leaking empty strings into Views

[asasemahmed]

What changed

The web app logs this error repeatedly:

Unexpected text node: . A text node cannot be a child of a <View>.

The "." is the end of React Native Web's message; the text node itself is an empty string. Conditions like {error && <Button …/>} render "" when error is the empty string, and the error states start as useState(""). So AgentStatus, which is on every screen, and the chat screen log it on every render.

String-valued conditions now use !!, so nothing renders when the string is empty. What shows when the string is non-empty is unchanged. That covers 13 sites in 7 files: error states, and optional strings such as task.result, monitor.lastValue, action.result, previewUrl, snapshot.message, and Sheet/LinkRow subtitle/detail.

Verification

• Counted the warnings while switching through Chat, Activity, Ideas, Goals and Apps in the web app:
  • main: 13 warnings.
  • This branch: 0.
• A scan of React fibers for Views with string children found the ChatScreen scroll content on main, and nothing on this branch.
• pnpm typecheck passes. Biome reports no issues for the changed files.
• pnpm test: 161 passed, 1 failed. The failure is Docker subprocess uses literal argv…, which fails on Windows with or without this change.

Integration limits

• Checked on web. On native, empty strings outside <Text> can throw "Text strings must be rendered within a component", so this should also remove that risk. I did not run a native build.
• Biome's lint/suspicious/noLeakedRender catches this pattern. It is not enabled, because it cannot see types: it flags 37 sites, mostly booleans that are already safe. I left enabling it to maintainers.