fix(profile): resolve replaced binaries and surface real launch failures

Two failures made a profile launch report the wrong cause.

Linux appends " (deleted)" to /proc/<pid>/exe once the running binary is
unlinked, which is what a google-chrome package upgrade does while the
browser stays open. detectRunningBrowser passed that literal string on as
the executable path, so every launch tried to spawn
"/opt/google/chrome/chrome (deleted)" and hit ENOENT. Executable paths are
now validated against disk, with the marker stripped and argv[0] used as a
fallback; a binary genuinely named "… (deleted)" still wins.

spawn() reports a missing or non-executable binary asynchronously via an
'error' event rather than throwing, and that event was discarded — so
openProfileWindow returned success, no window opened, and the caller
blamed the browser 15s later with a sentinel timeout. The launch now
awaits 'spawn'/'error', returns the real cause, and removes its handshake
page on the failure path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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