Fix SESSION_TIMEOUT_MS=0 and BROWSER_IDLE_TIMEOUT_MS=0 (0 = never)

[audinue]

Summary

Fixes the handling of `SESSION_TIMEOUT_MS` and `BROWSER_IDLE_TIMEOUT_MS` when their value is `0`.

Before

• `BROWSER_IDLE_TIMEOUT_MS=0` scheduled `setTimeout(..., 0)`, which shut the browser down immediately when idle — the opposite of the documented `0 = never`.
• `SESSION_TIMEOUT_MS=0` collapsed to the `600000` default via `parseInt(...) || 600000`, so sessions still expired instead of never.

After

• `0` now explicitly means disabled / never for both timeouts.
• Unset env vars still fall back to their defaults (session: `600000`, idle: `300000`).
• `scheduleBrowserIdleShutdown()` skips scheduling when `BROWSER_IDLE_TIMEOUT_MS <= 0`.
• The session-cleanup interval skips expiry when `SESSION_TIMEOUT_MS <= 0`.
• README updated so `SESSION_TIMEOUT_MS` documents `(0 = never)` to match `BROWSER_IDLE_TIMEOUT_MS`.

Verification

• `node --check` passes on `lib/config.js` and `server.js`.
• Parsing logic tested for unset / `0` / default-docs / custom values.

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