Refine sidebar project and session hierarchy

[touch2be]

Summary

• clarify project rows with folder semantics and compact session indentation
• use a stable leading-control slot that swaps the expand chevron for the drag handle without overlap or layout shift
• replace session color rails with subtle icon dots and keep the active state visually quiet
• turn the empty project state into a localized “New Session” action

Validation

• pnpm check
• git diff --check