fix: don't skip user presets when parent preset is not found

[BenJule]

Summary

Fixes #10869

Root Cause

In PresetCollection::load_presets() (src/libslic3r/Preset.cpp), when a user preset has an inherits field that points to a system preset that no longer exists (e.g. because it was renamed or removed in a system update), the preset is silently skipped:

// BEFORE – buggy
if ((inherits_config2 && !inherits_config2->value.empty())) {
    BOOST_LOG_TRIVIAL(error) << "can not find parent ...";
    continue;  // ← preset disappears from the UI entirely
}

From the user's perspective the preset appears to be gone after restarting the application, even though the file is still on disk. This explains why the workaround of recreating the User directory helps — it forces the user to re-create presets under the current system-preset names, so the parent is always found.

The bug can be triggered whenever:

• A system-preset name changes between releases (version bump, printer variant rename, etc.)
• The user's inherits reference still points to the old name

Fix

Instead of continue-ing (dropping the preset), fall back to the built-in default config and emit a warning log entry. The user's saved values are applied on top of the default, so their settings are preserved as well as possible. The preset is now visible in the UI and the user can correct any values that used to derive from the missing parent.

// AFTER – fixed
if ((inherits_config2 && !inherits_config2->value.empty())) {
    BOOST_LOG_TRIVIAL(warning) << "Parent preset not found; loading with default config as fallback.";
    // fall through to default config path — do NOT skip
}
preset.config = default_preset.config;
preset.config.apply(std::move(config));
extend_default_config_length(preset.config, {}, true, default_preset.config);

Behaviour change

Scenario	Before	After
Parent preset found	✅ loaded correctly	✅ unchanged
Parent preset not found	❌ preset silently dropped	✅ preset loaded with default base + user diffs, warning in log

Notes

The fallback may produce subtly different values for settings that were inherited (not explicitly saved) from the missing parent. This is a known trade-off: a preset with potentially imperfect settings is always preferable to a silently deleted preset.