feat(multipoint): restore channel settings when loading acquisition YAML

[hongquanli]

Summary

Dragging a previous acquisition (folder or acquisition.yaml) onto the Wellplate Multipoint widget restored only the channel selection and order — the per-channel exposure time, analog gain, and illumination intensity recorded in the YAML were ignored, so reproducing an acquisition still required re-entering them by hand. This PR restores those settings on drop.

Changes

• acquisition_yaml_loader.py: parse the full channels: section into a new ChannelYAMLSettings dataclass (exposure, gain, intensity). Values absent from older YAMLs or non-numeric parse to None and are simply not restored.
• AcquisitionYAMLDropMixin._restore_channel_settings: persist the parsed values through ConfigRepository.update_channel_setting() — the same path as live-control spinbox edits — respecting the current confocal/widefield mode.
  • Channels missing from the current configuration are skipped with a warning (consistent with the existing selection-restore behavior).
  • Out-of-range values (exposure ≤ 0, gain < 0, intensity outside 0–100) are skipped: update_channel_setting bypasses pydantic assignment validation, so persisting a bad value from a hand-edited YAML would make the whole profile fail validation on next startup.
• UI refresh: new signal_channel_settings_restored on both multipoint widgets, connected in gui_hcs.py to a new LiveControlWidget.refresh_current_mode_settings() that re-reads the active channel's values and re-applies the mode — without applying the live per-channel Z-offset, since the channel didn't change.

Deliberately not restored

Z-offset. z_offset_um is documented as sample-dependent (relative to the laser AF reference; "re-capture or reset when starting a new sample"), so importing an old sample's offsets could cause unwanted Z moves during acquisition.

The Flexible Multipoint widget gets the same signal for consistency, but its drop handler still shows "not supported yet" and never loads — no behavior change there.

Testing

• Two new loader tests: full nested channel settings; missing/partial/non-numeric values.
• New test_acquisition_yaml_channel_restore.py: restore + repo call args, confocal mode pass-through, unknown-channel / None / out-of-range skips, repo failure not counted, signal emitted only when something was restored.
• Related suites pass (166 tests: YAML loader, channel restore, channel sequence, config repo, live control offset).
• Note: 4 pre-existing failures in test_HighContentScreeningGui.py in my environment are unrelated — they fail identically on master (PyQt6 fallback vs the PyQt5-only activated[str] overload at widgets.py:4235).

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