[Feature]: Add project-wide replace with preview

[h3pdesign]

Problem

Project-wide search exists, but users still need a safe multi-file replace workflow inside the editor.

Proposed solution

Add project-wide replace with an explicit preview and confirmation step rather than blind replacement.

Acceptance criteria

• Users can review match groups before applying changes
• Replace can target a subset of results
• Changed files refresh cleanly in the editor and project tree
• The flow is safe for large projects and cancellable
• Keyboard and accessibility behavior remain predictable

[h3pdesign]

Implemented in v0.6.2 (project-wide replace with preview and selective apply/cancel flow). Closing for release.