fix(launcher): tolerate slow cold first start (Windows)

[lstein]

Problem

On a clean Windows machine, first launch timed out and the browser never opened. The server does start, but the first import torch is slow — Windows Defender scans the ~80 freshly-written package DLLs on first access, which routinely exceeds the launcher's old 60-second readiness wait. Subsequent launches are fast.

Fix

• Poll for server readiness up to 5 minutes instead of 60s. There's no penalty on the normal path: the browser opens the instant the port is reachable (typically seconds), well before the cap; the cap only bounds the "open it manually" fallback message.
• Abort the wait immediately if the server process exits (via a done channel), so a genuine crash no longer makes the user wait out the full timeout before seeing the error.

Tests

New browser_test.go: verifies it bails promptly on server exit and detects a reachable port quickly. gofmt/vet/-race clean.

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