Fix interactive service launch on system-python builds and surface failures

[mattdawkins]

Summary

The unified interactive service (segmentation + stereo) failed to launch on VIAME builds that use the system Python interpreter rather than a bundled one, and several failure modes degraded silently instead of being reported to the user. This addresses both.

Launch fixes

• bin/python fallback (linux.ts) — getViamePythonExe() hard-coded <viamePath>/bin/python, which only exists in bundled binary installs. From-source builds against system Python have no such interpreter (they rely on PYTHONPATH set by setup_viame.sh). It now uses the bundled interpreter when present and otherwise falls back to python on PATH. Windows is unchanged.
• -I → -s (interactive.ts) — the service was spawned with python -I, but isolated mode implies -E, which discards PYTHONPATH, so viame could not be imported (No module named 'viame'). Switched to -s (ignore the per-user site dir, but keep PYTHONPATH). The original intent of -I (avoid a stray cwd entry shadowing packages) is already covered because the process cwd is forced to viamePath, which has no shadowing modules.

Error surfacing

• Stereo enable launch failures (interactive.ts, api.ts, ViewerLoader.vue) — the stereo service watcher auto-runs on dataset load (not user-initiated) and previously degraded all failures silently so that uncalibrated datasets wouldn't pop a dialog. That also swallowed genuine service-launch failures. enable() now tags infrastructure failures with launchFailed, and the watcher always surfaces those in a dialog, while benign per-dataset failures (e.g. missing calibration) still degrade quietly.

Notes

• Stereo-segmentation match failures on the second camera (returning success: false with a clear message rather than an empty polygon) are handled in the corresponding viame.core Python service and are not part of this client diff.
• Build artifacts under client/.electron/ are gitignored and not included.

[BryonLewis]

Tested locally, mostly looks good to me.