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 37490167254

Coverage decreased (-0.005%) to 59.637%

Details

• Coverage decreased (-0.005%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 101 coverage regressions across 3 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

101 previously-covered lines in 3 files lost coverage.

File	Lines Losing Coverage	Coverage
mm/app/activeproject.cpp	64	71.0%
mm/app/layer/layerdetaildata.cpp	36	55.74%
mm/core/merginuserinfo.cpp	1	79.08%

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Coverage Stats

Relevant Lines:	15757
Covered Lines:	9397
Line Coverage:	59.64%
Coverage Strength:	93.99 hits per line

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💛 - Coveralls

[tomasMizera]

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

[Withalion]

The ternary operators as mentioned by Tomas in 2 places

[Withalion]

And fix formatting as well.

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	📬	Mergin Maps 73891 dmg	Expires: 04/01/2027	#7389
linux Build	📬	Mergin Maps 74151 x86_64	Expires: 04/01/2027	#7415
win64 Build	📬	Mergin Maps 65911 win64	Expires: 04/01/2027	#6591
Android Build	📬	Mergin Maps 870111 APK [armeabi-v7a]	Expires: 04/01/2027	#8701
	📬		Google Play Store	#8701
Android Build	📬	Mergin Maps 870151 APK [arm64-v8a]	Expires: 04/01/2027	#8701
	📬		Google Play Store	#8701
iOS Build	📬		Build number: 26.10.964111	#9641