feat: add Quick Start button for scratch sessions

[usvimal]

Summary

Adds a "Quick Start" button on the home page that opens a session in the user's home directory, allowing users to start working without selecting a specific project folder.

Use Case

• Quick experiments or prototyping
• General-purpose AI assistance
• When you want to "figure something out" but not as a project

Changes

• Added quickStart() function that opens session in home directory
• Added "Quick Start" button next to "Open project" (when projects exist)
• Added "Quick Start" button in empty state (no recent projects)

UI

Recent projects                    [Quick start] [Open project]
─────────────────────────────────────────────────────────────
~/project-1                                      2 hours ago
~/project-2                                      yesterday