Doc: Resolves #16603: Add documentation for conflict auto-merge and desktop resolution UI (#16604)

Co-authored-by: Laurent Cozic <laurent22@users.noreply.github.com>

readme/dev/spec/conflict_auto_merge.md

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+# Conflict auto-merge
+
+When the same note is edited on two devices before they sync, if a change is made when one of the devices was not fully synced, Joplin creates a conflict note. Auto-merge avoids this when the edits don't overlap, using a three-way merge. It runs on the client that syncs second; other devices just receive the merged note.
+
+## Base version
+
+The merge needs three versions: **base** (the last version this device and the sync target agreed on), **local** and **remote**.
+
+The base is stored per sync target in `sync_items.base_body` and `base_title`. It is recorded after an upload, after a download, and after a conflict, whichever version is left in the original note. If a downloaded note is encrypted, the base is cleared until it is decrypted, so that a conflict before decryption creates a normal conflict note.
+
+## Merge
+
+The merge is line-based. Regions changed by only one side are taken as-is. Regions changed by both sides are auto-merged if both made the same change, otherwise they become a conflict. Edits on the same or adjacent lines therefore conflict. Word-level merging was tried and dropped because it could silently duplicate text. Titles follow the same rule.
+
+If everything merges, the merged note is saved over the local note, becomes the new base, and its `updated_time` is moved past the remote time so it gets uploaded. If some regions conflict, a conflict note is still created, but both notes already include the non-conflicting changes, so they differ only where there's a real conflict.
+
+## Fallback to a conflict note
+
+The merge is skipped, and a normal conflict note is created, when:
+
+- `sync.autoMergeConflicts` setting is off
+- there is no recorded base
+- the item is read-only or either note is locked
+- either note cannot be decrypted
+- an edit which touches identical lines, since the diff can't tell which one changed and may duplicate content.
+- the diff exceeds its bounds (`maxEditLength` 5000, timeout 1000ms)
+
+## E2EE
+
+An encrypted remote note is decrypted in memory for the merge without being saved. If a conflict note is created, the original note is saved with the decrypted (and partially merged) remote content.
+
+## Code
+
+- `services/conflict/diffNotes.ts`: merge engine (`autoMerge()`) and the duplicate-line guard
+- `services/conflict/boundedDiff3.ts`: fork of `node-diff3`'s `diff3MergeRegions()` using a bounded diff
+- `services/conflict/autoMergeNote.ts`: merges title and body of a note
+- `services/synchronizer/utils/handleConflictAction.ts`: runs the merge during sync and applies the result
+- `models/BaseItem.ts`: stores the base and keeps it when a sync doesn't provide a new one