feat(desktop): distinguish branched conversations with numbered title suffixes

## Problem

When branching a conversation, the new conversation inherits the original title without any distinguishing suffix.

Creating multiple branches therefore produces several identically named conversations in the sidebar. Users must open them individually or rename them manually to distinguish the original conversation from its branches.

## Expected behavior

Automatically append a numbered suffix when creating a conversation branch. For example:

- Original: `Review project`
- First branch: `Review project (1)`
- Second branch: `Review project (2)`
- Third branch: `Review project (3)`

The original conversation's title should remain unchanged, and users should still be able to rename any branch manually.

## Suggested naming rules

- Choose an unused numeric suffix to avoid collisions with existing conversation titles.
- When branching an already numbered branch, avoid stacking suffixes such as `Review project (1) (1)`.
- Apply the naming behavior consistently across all ordinary conversation branching entry points.

## Scope

This request concerns only the initial title assigned to an ordinary conversation branch.

It is separate from automatic title generation for new conversations and does not require changing conversation history or branch relationships.