Add a workflow for merging duplicate wiki pages

[catherineluse]

Problem

Alternate names prevent new duplicate wiki pages from being created when a character or location is known by multiple names, but existing projects may already contain multiple pages for the same entity.

Users need a safe way to choose a canonical wiki page and merge duplicate pages into it without losing related data.

Proposed workflow

• select a canonical destination page and one or more duplicate source pages
• preview the data that will be combined before applying the merge
• combine or explicitly resolve conflicts in page content, summaries, tags, and alternate names
• move chapter/wiki mentions and source-chapter links to the canonical page
• move wiki update history and contradiction notes
• move illustration tags, image associations, and cover references
• preserve the source page names as alternate names on the canonical page
• delete the source pages only after the merge completes successfully
• make the operation transactional where supported, or provide equivalent rollback safety

Acceptance criteria

• users can initiate a merge from a wiki page
• aliases and canonical names are checked for conflicts before merging
• all relationships point to the surviving page after the merge
• duplicate links are removed
• no images, update history, or source-chapter relationships are lost
• the UI clearly previews destructive effects and requires confirmation
• web, Electron, and Capacitor storage paths behave consistently
• automated tests cover content conflicts, relationship migration, alias conflicts, and rollback/failure behavior

Context

This is intentionally deferred from the alternate-name feature. Alternate names solve future identity resolution; this issue addresses consolidation of duplicates that already exist.