Stop default participant preselection in Add Expense

[Copilot]

When creating a new expense, the form preselected all group members in the split, which made accidental broad splits likely. This change makes participant selection explicit by starting with no selected members.

• Behavior change: new expense defaults

  • Updated AddExpense initialization to set selectedMembers to an empty list for create flow.
  • Edit flow remains unchanged and still hydrates participants from existing splits.

• Submission guardrail

  • Added validation in computeSplits() for non-line_item modes to reject submits when no members are selected.
  • Returns a clear form error instead of creating an expense with empty splits.

• Key code change

  if (!isEdit) {
    setSelectedMembers([])
    setPaidBy(profile?.id)
  }
  
  if (splitMode !== 'line_item' && selectedMembers.length === 0) {
    return { splits: [], error: 'Select at least one member' }
  }

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