Add opt-in text conflict automerge

[msexxeta]

Context

Follow-up for https://github.com/orgs/opencloud-eu/discussions/2891.

This adds an opt-in client-side automerge path for text-file sync conflicts. The companion Craft dependency PR is opencloud-eu/craft-blueprints-opencloud#6.

Changes

• Add a persisted autoMergeTextConflicts setting and checkbox in General Settings.
• Add localized strings for the new setting across the existing desktop translation catalogs.
• Add libgit2 discovery/linking for libsync.
• Add a text conflict automerge helper that:
  • only runs for enabled text-file conflicts with a valid synced base journal record;
  • uses the server versions endpoint for the prior synced base version;
  • performs a three-way merge with libgit2 using Qt text-mode I/O for platform newline handling;
  • falls back to the existing conflicted-copy behavior when merge setup or merge execution fails.
• Wire the automerge attempt into the conflict download path before the normal conflict-copy fallback.

Verification

• Enabled the setting in the active local config and confirmed autoMergeTextConflicts=true.
• Used a logged-in OpenCloud web UI session only as an auth source for WebDAV verification; no discussion post was made.
• Provoked a real sync conflict on Codex Sync Tests/automerge-webui-20260605-1327.txt:
  • base synced first;
  • local edit changed line 3;
  • remote WebDAV edit changed line 1;
  • client log showed Automatically merged text conflict;
  • no conflicted copy was created for that successful test;
  • local file and authenticated WebDAV GET both returned:

remote line 1
base line 2
local line 3

• Rebuilt translations with lrelease; updated .qm generation completed successfully.
• Cleaned the local Craft opencloud/opencloud-desktop build tree and blueprint __pycache__, then rebuilt through Craft with the repo .craft.ini plus workflow override using -i --install-deps --compile --install.
• CMake configure found required LibGit2 from the Craft prefix.
• Craft compile completed successfully (450/450), install succeeded, and qmerge succeeded.
• Verified the qmerged OpenCloudLibSync.dll depends on git2.dll with dumpbin /dependents.

[msexxeta]

I have had this running for a few days now and noticed a memory leak, just fyi. I will investigate and attempt to fix it.