Fix app crash on selecting unavailable layer, Add info message

[xkello]

Description

Tapping a layer that failed to load (bad PostGIS credentials, offline server, missing local file, etc.) crashed the app instead of telling the user anything useful.

Fixes: #4568

What changed

• LayerFeaturesModel::populate() now checks isValid() before querying the layer's data provider. This was the actual cause of the crash, since a broken connection still has a non-null dataProvider().
• LayerDetailData gained an isValid property so QML knows whether the tapped layer actually loaded, and logs the real failure reason internally (via CoreUtils::log) without exposing it to the user.
• MMLayerDetailPage.qml now shows a generic "Layer unavailable" screen (with a normal header/back button) instead of building the features list and legend when the layer is invalid.
• ActiveProject::validateProject() now logs the underlying error reason alongside the existing Project Issues entry, for easier debugging.

Behaviour

Before: tapping an unavailable layer crashed the app.
After: the user sees a "Layer unavailable" message and can navigate back normally; the specific reason (auth failure, offline, missing file, etc.) is only written to the log.

Screenshots

Before	After

[github-actions]

Coverage Report for CI Build 36016602754

Coverage decreased (-0.01%) to 59.501%

Details

• Coverage decreased (-0.01%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 93 coverage regressions across 2 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

93 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
mm/app/activeproject.cpp	58	71.04%
mm/app/layer/layerdetaildata.cpp	35	57.14%

---

Coverage Stats

Relevant Lines:	15751
Covered Lines:	9372
Line Coverage:	59.5%
Coverage Strength:	94.01 hits per line

---

💛 - Coveralls

[tomasMizera]

Looks good, thanks! I left a minor comment about ternary operators. Once fix, please proceed to testing without my review :)

[tomasMizera]

Let's not nest multiple ternary operators. They become quite hard to read.

Try comparing it with this

Suggested change

	QgsError layerError = layer->error();
	const QString reason = !layerError.isEmpty() ? layerError.summary()
	: ( layer->dataProvider() ? layer->dataProvider()->error().summary() : QString() );
	CoreUtils::log( QStringLiteral( "Project load" ), QStringLiteral( "Invalid layer %1: %2" ).arg( layer->name(), reason ) );
	QgsError layerError = layer->error();
	QString reason;
	if ( !layerError.isEmpty() )
	{
	reason = layerError.summary();
	}
	else if ( layer->dataProvider() )
	{
	reason = layer->dataProvider()->error().summary();
	}
	CoreUtils::log( QStringLiteral( "Project load" ), QStringLiteral( "Invalid layer %1: %2" ).arg( layer->name(), reason ) );

[tomasMizera]

I believe our code convention says that there can be only one ternary, not multiple ones nested.

[Withalion]

The ternary operators as mentioned by Tomas in 2 places