feat: add new session confirmation dialog with Esc-to-back and state preservation

[huang-hf]

Summary

• Add a confirmation step before creating a new session
• Fix: Esc on confirmation screen returns to form instead of closing the dialog
• Fix: Form state is preserved when navigating back from confirmation

Changes

New: Confirmation dialog before session creation

When the user presses Enter to create a session, a confirmation screen is shown with a summary of the current configuration (tool, title, path, branch, etc.). The session is only created after the user confirms.

Fix: Esc returns to form

Pressing Esc on the confirmation screen now returns to the form for further editing, rather than closing the entire new session dialog.

Root cause: DialogProvider's keyboard handler fires before DialogNew's, so Esc was popping DialogNew itself off the stack entirely.

Fix: Form state preserved on back

All form values (tool, title, path, branch, worktree options, etc.) are fully restored when navigating back from the confirmation screen.

Root cause: dialog.push() unmounts DialogNew (only the top stack item is rendered by DialogProvider). When dialog.pop() is called, a new DialogNew instance is created with default signal values, losing all user input.

Solution: Before calling dialog.push(), all form signal values are saved to a module-level _savedFormState variable. When DialogNew mounts, it reads and restores from this variable, then clears it.

Test plan

• Fill in the new session form (tool, title, path, branch, etc.)
• Press Enter → confirmation dialog appears with correct config summary
• Press Esc → returns to form with all values intact
• Press Enter again → confirm creates the session successfully
• Press Esc on the main form → closes the entire dialog (unchanged behavior)

🤖 Generated with Claude Code

[huang-hf]

Replacing with a clean branch that doesn't include unrelated copyClaudeDir changes.