Decouple selective sync

[tomasMizera]

Removes old spaghetti code for selective sync, including the logic that suppressed it via setSupportsSelectiveSync on MerginApi.

The supportsSelectiveSync flag is gone, replaced by a per-project file flag indicating whether selective sync should be used. This simplifies testing: a test client can mimic server logic and update mergin-config.json, which is normally forbidden on mobile.

The flag lives at .mergin/selective-sync.ignored, an empty file. If present, selective sync is ignored for that project. It's written on first download, then read automatically - MerginApi no longer needs to be told whether to ignore selective sync.

As a side effect, hasLocalChanges no longer depends on MerginApi's internal state; it now reads the setup from the file directly.

Managed to make tests compilable, selective sync autotests are nearly passing. I will fix those first before merging the PR.

Fixes one of the points in #3959

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	❌	Build failed or not found.		#7338
linux Build	❌	Build failed or not found.		#7364
win64 Build	📬	Mergin Maps 65401 win64	Expires: 24/12/2026	#6540
Android Build	❌	Build failed or not found.		#8650
iOS Build	📬		Build number: 26.09.959011	#9590